Yep, I've spent ages with the seats in the car, practicing steering, changing gear etc at different heights. I was planning on bolting straight to the floor, and 5-6 inch felt about right.
But the latest plan is to use the subframe from the old seats - which will make the seat a little higher, but will allow them to adjust back and forward etc.
With the additional height, I might not need to recline them so much, so may be able to have around 4in.