As has been said, it was in GT4.
What? The OP said he liked to use it on small, gutless cars to corner faster, no amount of skill will change the fact that a 50bhp Daihatsu Midget will lose in a race against a 300bhp Mitsubishi Evo, but if you could add 100bhp temporarily then it would at least be a closer race.
I only ever really used nitrous in my fully upgraded Evo VI RS TME and VIII GSR MR for Tsukuba time attacks. I shortened the ratios so that they'd top out at around 120mph, upped the AYC a fair bit, put racing softs on and turned the nitrous all the way up, so essentially they had 600bhp for approximately half a lap. Seriously, you had to be very, very quick to react when driving them and you were changing gear pretty much all the time, they were so much fun, but in GT5 you can't replicate that. I think I had them in the mid-high 40's for a lap back in 2007, maybe I'm remembering that wrongly. I honestly seem to remember a high 46 second lap in the VIII MR, with nitrous.
In summary: Don't be so uptight, nitrous oxide isn't exclusively limited to ricers and can offer massive performance gains, why would you actually want fewer options in GT5? If you don't want to use it, you don't have to, but I don't see why snobbery should dictate that it stays out of the game.