Hope it's an easy fix!
Did a leakdown test and then replaced the coils and plugs on the S2000...
Test results:
Cyl 1 - 2%
Cyl 2 - 15%
Cyl 3 - 5%
Cyl 4 - 2%
Not great for Cyl 2, hoping that it is simply a valve adjustment or something simple that is needed.
The plugs haven't been replaced in a while, I thought the previous owner did them not too long ago but guess not - they were gapped super tight (NGK BKR8EIX but gapped to like 0.29? which is crazy tight considering OE gap is 0.39-0.41) which implied they were the plugs from the big boost setup that was on there 2 owners ago. Swapped them with new BKR7EIX-11s gapped to 0.39, and new Delphi coil packs. Seems to help the car run a bit smoother and fiddled with the K-Pro map to help with idle/low speed issues.
Going to have to spend more time investigating what the heck is going on with Cyl 2 but the car drives well so far...
EDIT: should say the plugs looked OK, nothing too obvious with them except that the gaps were super tight (which may explain the rough idle)