Would an S2000 with only a Drop in K & N filter and custom test pipe benefit from a tune on KPRO? Not necessarily for peak numbers but can it improve the powerband or would I be better off staying with a stock ECU? Only reason I am asking is cause I own a RSX with KPRO already and wanted to give the S some more pep.

 

it costs a couple hundred to convert an rsx kpro to an s2000.
it costs additional couple hundred to buy the sensors, valvecover, and install so that the modified kpro works with the s2000
a stock s2000 will have benefits with a kpro or any other tuning system; the kpro will probably benefit your DC5 more than the s2000 the way it is now.

since you have a MY03, a VAFC will do more per dollar given your mods.

 

You will only tune effectively WOT (open loop) with a VAFC. Partial throttle (closed loop) would be clamped at 14.7AFR since that's all the stock narrowband can read essentially overriding any of the VAFC trickery. You will not be able to independently tune ignition timing with a VAFC.