NOS octane booster adds a few points of octane, yes... but it doesn't make 91 octane gas into 95 octane, it actually changes 91 int more like 91.5 (or was it 91.05??). Their use of "points" is
HIGHLY HIGHLY HIGHLY misleading.
That said, I'd follow ^his^ suggestion and NOT try it. it's a bleeding waste of money.
As for octane too high... I've run straight 100 (which isn't as high as one could go) in my VW GTi (1.8Turbo) and it felt absolutely GREAT, despite peoples' telling me it wouldn't make a difference (since the car was 'tuned' for 91 here in ca/93 elsewhere). I swear it pulled harder with 100 in there.
I think with higher octane gas you will also see a decrease in gas mileage. (or at least that's what I noticed.)
Were our KA24DEs made for 91??! i was told (was under the impression) that it was made for 87/89 octane... anyone still have their instruction manual? <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/confused.gif" border="0" valign="absmiddle" alt='???'>