I do think that a signalling model of education

Once again, which education? Clearly, a training course for, say, a truck driver, is not signalling, but exactly what it says on the can: a training course for driving trucks. A language course, likewise so. Same goes for mathematics, hard sciences, and engineering disciplines. Which may perhaps be likened to necessity of training for a formula 1 driver, irrespective of the level of innate talent (within the human range of ability).

Now, if that was within the realm of actual science, something like this "signalling model of education" would be immediately invalidated by the truck driving example. No excuses. One can mend it into a "signalling model of some components of education in soft sciences". Where there's a big problem for "signalling" model: a PhD in those fields in particular is a poorer indicator of ability, innate and learned, than in technical fields (lower average IQs, etc), and signals very little.

edit: by the way, the innate 'talent' is not in any way exclusive of importance of learning; some recent research indicates that highly intelligent individuals retain neuroplasticity for longer time, which lets them acquire more skills. Which would by the way explain why child prodigies fairly often become very mediocre adults, especially whenever lack of learning is involved.