If your track is smooth as marble and you never put a wheel off, then I’m sure you can get away with mounting it 5mm below. However I typically go pretty much flush with the rails to keep from blasting the bottom out of an expensive seat. I also despise fitting seats so outside of the expense I just don’t want to do it twice.
You will find a tick or two below frame rails will make a difference on weight transfer when you are 6’+. Another way to help is to tilt the seat back a bit. I have the front even with the frame rails and the rear flat a tick below (2-3mm). I find this helps get my CG down a little without dragging the seat bottom around the track. Of course track surface will have an effect. Super bumpy or being prone to dropping a wheel will shorten the seat life.
I had to play with seat position a lot in my otk with tillet seat and ended up just a tick above the frame rails but had to have the seat a bit further forward than otk specs to get it to work right. Evinco blue tires fairly low grip track in a 80cc shifter kart.