If the above is true, then what is there left to explain ?

Radically unfamiliar-to-humans qualia. You have picked an easy case, I have picked a difficult one. If we wan't to know what the world sonars like to a bat on LSD, identity theory doens't tell us.

Except that there is, since there are plenty of subjects which have been studied from both sides. The natures of space, time and causality for a start.

If I can jump in... It's useful to distinguish between phenomenology in general, as the study of consciousness from "within" consciousness; various schools of phenomenological thought, distinguished by their methods and conclusions; and then all those attempts to explain the relationship between consciousness and the material world. These days the word "phenomenology" is used quite frequently in the latter context, and often just to designate what it is that one is trying to "correlate" with the neurons.

It's part of the general pattern of usage whereby an "-ology" comes to designate its subject matter, so that "biology" means life and not the study of life - "we share the same biology" doesn't mean our biology classes are in agreement - "psychology" means mind and not the study of mind, and "sociology" means social processes and not the study of them. That's an odd little trend and I don't know what to make of it, but in any case, "phenomenology" is often used as a synonym for the phenomena of consciousness, rather than to refer to the study of those phenomena or to a genuine theory of subjectivity.

Thus people talk about "naturalizing phenomenology", but they don't mean taking a specific theory of subjective consciousness and embedding it within natural science, they just mean embedding consciousness within natural science. Consciousness is treated in a very imprecise way, compared to e.g. neuroscience. Such precision as exists is usually in the domain of philosophical definition of concepts. But you don't see people talking about methods for precise introspection or for precise description of a state of consciousness, or methods for precise arbitration of epistemological disputes about consciousness.

Phenomenology as a discipline includes such methodological issues. But this is a discipline which exists more as an unknown ideal and as an object of historical study. Today we have some analytic precision in the definition of phenomenological concepts, and total imprecision in all other aspects, and even a lack of awareness that precision might be possible or desirable in those other aspects.

Historically, phenomenology is identified with a particular movement within philosophy, one which attached especial significance to consciousness as a starting point of knowledge and as an object of study. It could be argued that this is another sign of intellectual underdevelopment, in the discipline of philosophy as a whole - that phenomenology is regarded as a school of thought, rather than as a specific branch of philosophy like epistemology or ethics. It's as if people spoke about "the biological school of scientific thought", to refer to an obscure movement of scientists who stood out because they thought "life" should be studied scientifically.

So to recap, there is a movement to "naturalize phenomenology" but really it means the movement to "naturalize consciousness", i.e. place consciousness within natural science. And anyone trying to do that implicitly has a personal theory of consciousness - they must have some concept of what it is. But not many of those people are self-consciously adherents to any of the theories of consciousness which historically are known as phenomenological. And of those who are, I think there would be considerably more enthusiasm for "existential phenomenology" than for "transcendental phenomenology".

This distinction goes back to the divide between Husserl and his student Heidegger. Husserl was a rationalist in an older, subjective sense and by temperament - he was interested in analytical thought and in the analytical study of analytical thought; the phenomenology of propositional thinking, for example. Heidegger was his best student, but he became obsessed with the phenomenology of "Being", which became a gateway for the study of angst, dread, the meaning of life, and a lot of other things that were a lot more popular and exciting than the intentional structure of the perception of an apple. The later Heidegger even thought that the best phenomenology is found in the poetic use of language, which makes some sense - such language evokes, it gets people to employ complex integrated systems of concepts which aren't so easy to specify in detail.

Meanwhile, Husserl's more rationalistc tendencies led towards transcendental phenomenology, which even among philosophers was widely regarded as misguided, the pursuit of a phantasmal "transcendental ego" that was (according to the criticism) an artefact produced by language or by religious metaphysics. Husserl literally fled Nazi Germany in order to continue his work (while Heidegger tried to accommodate himself to the sturm und drang of the regime) and died with only a few loyalists developing the last phase of his ideas. After the war, Heidegger was excoriated for his politics, but existential phenomenology remained culturally victorious.

If we come closer to the present and the age of cognitive science, there are now many people who are appreciative of Husserl's earlier analyses, but transcendental phenomenology is still mostly regarded as misguided and metaphysical. Existential phenomenology is also a somewhat exotic affiliation among scientists, but it does get some recognition among people who are into the importance of "embodiment" in cognitive science and consciousness studies. Husserl's phenomenology is so verbal and verbalizing, whereas existential phenomenology, in its attention to "raw existence", can lead (among other destinations) to a 1960s-style rediscovery of the senses, e.g. in Merleau-Ponty's phenomenology, and from there to the embodied cognition of Rodney Brooks et al.

So in the contemporary world, transcendental phenomenology is very obscure and mostly it's a subject of historical research. You could make the analogy between Husserl and Einstein, with transcendental phenomenology as Husserl's unified field theory. Einstein was regarded as a founder of modern physics but his later interests were regarded as misguided, and it's much the same with Husserl. But fifty years after Einstein's death, unified theories are a standard interest, it's just that they're quantum rather than classical. Similarly, it's likely that the spirit of transcendental phenomenology will be revived eventually.