My personal observations from fitting the RX7s on the race car...
Rotor thickness is the same, but isn't really an issue as they are vented. +20mm diameter gives a big increase in surface area - as its a circle.
RX7 pads look to have bigger surface area than E30 pads - yes they are small in relation to the calipers.
Got my 15" weaves over the calipers using a 5mm spacer, many designs of 15" wheel clear the calipers without need for any spacer depending on the offset.
4 x pistons in the RX7 versus the one on the E30, much better to have equal pressure on both sides of the disc than one piston and sliding caliper. I would say the balance on my E30 is now better with the RX7 fronts than it was with the standard E30 set-up.
Over-all it is a very good bang for buck upgrade of the E30 front brakes, better bite (without being grabby), good balance, etc. I am running Hawk HP+ pads front and rear which probably contributes as well. I'm sure there are better brake set ups for the E30, but at much greater cost. Plus pads and rotors are easily obtainable for this swap, not hard to get dedicated Wilwood / Brembo / AP Racing parts.