Seeing as people want to know:
The leather that Toyota used through the years is, frankly, crap. It does like to stretch and, after a while, the seats look crap as well.
Alcantara does have a small amount of stretch in it, but only on one axis (whereas leather stretches in all directions). So using it on seat centres will lead to less sagging than using Mr T's leather.
There's always a downside though. No matter what anyone tells you, Alcantara is simply not as durable or robust as leather is. So it will wear faster and it will look a bit tired faster!
As for "colouring the stitching"... well, let's just say I use expensive thread because I want it to be colour-fast in all conditions...