Tulpas and other such experiences seem plausible given how prone we are to hallucinating things anyway (see intense religious experiences for example), and I wouldn't be surprised if some people would be able to create them consciously. However I doubt that most people can do this. The regulars of /r/tulpas are probably not very representative of the population at large, whether through their unusual proficiency with mental imagery or some deeper eccentricity.
Creating a tulpa in order to develop skills faster or become more productive might work, but the question is whether the gains weighted by probability of success are higher than other, more conventional (and indeed, mentally healthy) methods. I think not.
On the topic of religious experiences, I found this bit from the linked tulpa FAQ very interesting:
...By talking and fleshing out something to your own subconscious for so long, you start to receive answers from them. The answers will tend to align themselves with all the preconceived traits you give them. The answers you get may surprise you, and in doing so show independent sentience. This sentience can be thought of as the "core" of the tulpa. The rest is just building a form in your mind for them to take, allowing for deviation of that form, a
Thus spake Eliezer:
It seems that many here might have outlandish ideas for ways of improving our lives. For instance, a recent post advocated installing really bright lights as a way to boost alertness and productivity. We should not adopt such hacks into our dogma until we're pretty sure they work; however, one way of knowing whether a crazy idea works is to try implementing it, and you may have more ideas than you're planning to implement.
So: please post all such lifehack ideas! Even if you haven't tried them, even if they seem unlikely to work. Post them separately, unless some other way would be more appropriate. If you've tried some idea and it hasn't worked, it would be useful to post that too.