In the concept's picture, from the RX-Evolv :)

http://www.rx7-gallery.co.uk/binrx8/rx8pics/rxevolv_17.jpg

That's a lot of rear seat room :)

looking at the front seat design in a few of the concept RX-8 pics, they look bulkier than those tubular frame ones in the RX-Evolv... so maybe the rear seat leg room is cut by a few cm's...

But remember, the Rx-8 has a longer wheelbase than the 626 even tho it is much smaller, leaving more space for "living quarters". Plus, the seats still are pretty thin and the rear seats are recessed into the back of the car. I dont think rear seat comfort will be compromised.