On my Seat Leon Cupra R the best mod was the remap, that was mad, 275bhp from the 1.8T. I didn't need to do anything else to it as it already had the body kit, wheels and Brembo brakes. I miss that car.
On the IS220D, has to be the new wheels, they look awesome, not sure about the new springs, I think I might have spoiled the ride for the sake of looks.
The 928 is an ongoing thing, as would be with an 18 year old supercar. I think the best was not so much of a mod really, but replacing the ageing suspension with new components restored the cars handling, and stopped it banging the nose spoiler on the ground over speed humps, it was a bit wobbly. I have fitted a new RS steering wheel and am in the middle of replacing the centre exhaust section with a cross over through pipe, which adds some power and frees up the breathing. I am yet to drive it, but from other owners comments, it will be louder and faster in the mid range.
Some people have fitted wood trim to the interior, and I for one will be staying away from that route of modification. I may in the future consider importing a supercharger kit from the states. Oh I nearly forgot, actually the best mod was fitting a LSD as that really helps get the power down, but also have to be really careful in the corners!