I thought it was well known that there was gains to be had from an exhaust cam gear on R33's stock cams, aftermarket ecu or not. You retard it up to 4 degrees.
Generally cam gears can be used to change the characteristics of the power. Ie you can swap some top end for earlier boosting and more mid range power. It's up to you, the tuner and the gains/losses involved whether this is worthwhile for you.
I have 260 odd rwkw and I think I would be happy with another 100rwkw but then again I said that about 250rwkw when I was at 190.
As for specs, they all do the same thing. Get a cheap one I say.