I think this depends how tall you are. We have the isofix base set up behind the drivers seat with a Maxi Cosi Cabriofix, and although the seat is snug to the back of my seat I am not restricted by it, but then I sit quite close to the steering wheel as I have short legs (5'11" though)
But, this is with one 5 month old, so I would think it would be impractical with a second child as the boot is good but mostly taken up with the Quinny Buzz frame and seat for the youngest (in our case).
I bought mine with the intention of upgrading the wife's to a more practical car, probably a C-SUV sometime when they drop in price.
EDIT: To sum up, I think I agree with the op. At least with the TI you can drop the back seats down for going to the tip/IKEA though (yes, the tip and IKEA are interchangeable in that sentence!)