i think overall, honda engines do require better octane, d-series, h-series or b-series. Ive found my d-series runs less kms on 91 than on 95 or 98 for that matter. The car seems more sluggish with 91, im not so sure why, but it does feel alot healthier on premium fuels. I do think 98 is over kill on these engines, but as pointed out, there are more benefits than just knock resistance to be gained.

As far as 100 octane goes, unless the engine was built for that, forget it LOL. Only minority ppl use it, mostly Skyline GTR owners as such. But its been proven that 98 octane can be safely tuned on cars, an making close to 600hp, so i dont see why u really need to run 100 octane.