It's not like you can take the hubs/uprights/suspension from a miata and just stick it on a brz. For one the design and layout is completely different especially up front.

But if you really want a light, 2-door, rwd car that uses 4x100 miata hubs and brakes I can think of a good way to do that...

Both, but tire construction definitely has an effect. My summer and winter tires are the same overall diameter and rim size (225/45/47 and 205/50/17), there's a big difference in feel. I've also driven the same tire back to back in different sizes (225/47/17 and 225/40/18) and it's noticeable too. Not as much as snows vs summers, but noticeable. There's also a difference in feel between similar tires in the same size - RS3's vs ZII's vs AD08R's all feel different. ZII's and AD08R's are more similar to each other than to the RS3's, but are still subtly different.

This does seem to be Miata Is Always The Answer week on FT86club!

Miata is wonderful car, but 1) i don't fit in it, 2) it's practicality is slightly below my acceptable treshold for only car, while twins are usable enough, with normal roof, extra seats, size, etc. And if looking to track only toy, all practicality/usability aside, why not go further, to something like lotus7 / ariel atom / ktm crossbow?