dont know if it is the same with rb20's but with 25's the cc is a rough guide to hp at the wheels, so 550cc should get 550hp at the wheels @ 100% duty, you really only want to see 80% duty tho so realistically 550cc should be good for a safe 440hp atw, the 300 your looking for you should go 350-400, i would suggest 450-500 tho as the power bug will bite you and they give you a bit of room to play with.
I believe gtr injectors are 440cc and with a balast resistor are a straight bolt in for rb20's, add an adjustable fuel pressure reg and they should be good for near on 500cc and should be cheap, depending on the price of the balast resistor.