I've noticed that even when I buy two different pasta shapes of the same brand (and so presumably the exact same recipe), they still taste subtly different, to the point that I consistently prefer the tastes of certain shapes of pasta. Is there any basis for this, or am I experiencing some form of synesthesia?
It will be slight variations in the recepies, slight variations in how you cookied it or it will be your mind playing tricks. Shape will not affect the taste of pasta. One shape however may be thicker than others, and if not cooked taking that into account, you may have a different taste, but cooked to the same level, no difference.
Different forms of pasta are going to have different surface-to-mass ratios, which will affect how the pasta cooks; the cross-section may affect how much starch gets rubbed off as the pasta boils. (I can only assume that more's going to stay in a spiral or a tube than will on something that can rub up against other pasta)
But I'm going to assume that you're not eating the pasta plain ... and the different shapes will hold sauce differently, and that can be quite significant.
The other thing that Tom Gullen mentioned is variation in cooking -- I personally avoid capellini (aka angel hair), because I've had it overcooked so many times ... overcooked pasta is disgusting, in my opinion.
Taste is a factor of not only teste buds on tongue and roof of mouth but also of sight, smell, and Texture or feel which is influened directly by shape.
Pasta tastes the same and is the same no matter what shape it is. As someone stated above it can depend on how thick the pasta is and how long you cook it.