I've had a different experience tuning with 95 vs 98.
Granted it was with a different engine but it had a very similar compression ratio (8.4:1 iirc), slightly higher dynamic compression (180 psi) and ran a gt35r, a 0.82 rear housing and a large external gate.
3 octane points was not insignificant and allowed for 20% more timing.
If you see little to no gain using 98 over 95 then I suspect your car has been tuned for 95 or runs a super safe 98 tune that pulls timing regularly when run on less than 98.
This is often the case when you pay for a tune. No sensible tuner wants a car at the ragged edge. Go look at jets tuning thread to see how he felt about taking a stock motor over 500rwkw despite his ability to control knock and fuel. Yes that example is timing vs mechanical but you get the drift.
Baring faulty parts a misfire at idle is caused from either; running lean, a too small/insufficient plug gap, incorrect cam timing at idle or too much ignition timing at idle. Bad fuel will cause a misfire at idle too but that seems highly unlikely in your case.
Oh I almost forgot, water out of suspension can create intermittent misfires.
Fwiw I could run as lean as 15:1 at idle with about 7 degrees btdc but needed 13.7:1 to run 15-20 degrees btdc without misfiring in my other car.
I tune my ford to the limits of 98 and would not consider using 95 unless 98 was not available.
Sorry for the novel fellas but hey, it might come in handy for someone one day.