Don't forget to let the i30n owners know in advance that you will only be able to race in 10second intervals ... 
Not necessary as with the exception of the Autobahn, you won't be able to use the throttle for 10 sec non stop on any public road. Definitely not possible in most tracks either. And by the way, the moment you lift off the overboost counter resets so...
Also, BMS dyno'ed a Clubsport with the overboost on and off, the difference was less than 10hp and always over 280

@OP: If it was pulling away so fast in a straight line then most definitely it was mapped or running a piggy. Despite the extra bhp the in-gear acceleration numbers of the i30N in tests were not as impressive as the numbers suggested so it should have been much closer. If this had been on a twisty b-road I'd say it would be normal as even stock it should be able to carry more speed with a half decent driver. But 2.0 TSIs have great delivery and generally punch a bit above their weight.
PS. If you worry about performance, as others have said a stock GTI is nothing impressive by today's standards. Has excellent potential though.