When I got my ex-MOD 90 I fitted Recaro's (at the time JC was selling them brand-new / take-out for £100 a pair which was too good to miss) and they were excellent however after about 8-years I decided I wanted the standard look so sold them and invested in some Tracker replacements which while not as comfortable or supportive are OE fit.
The problem with any aftermarket seat is the mounting, thickness of the base and underseat access.
Mounting the seats wasn't a problem, I had steel sheets cut which fitted the seatbox and then used rivnuts in the seatbox to hold the plate. Studs welded through the plates provided secure mounting for the seat-runners.
Thickness of the base wasn't an issue for me as I also have a smaller wheel fitted but it did mean my wife couldn't dip the clutch pedal so it restricted her use of the vehicle.
Access to the underseat areas wasn't a real issue as I always fit a battery isolator and the use of captive nuts & studs mean that I could unbolt and remove a seat in the time it took to undo four nuts from captive studs and the plate in the time it took to remove eight bolts from captive-nuts (i.e. very quickly).
Comfort was excellent. I once drove 400-miles to my destination stopping only once for fuel, slept in the drivers seat for a few hours and then drove home again only to do the whole thing again 3-days later!