Skyline GT-R / tuning for more HP, less lag?

Hey, I'm no car expert so just need a little bit of advice on car tuning. I've just bought a r33 '97 Nissan Skyline GT-R twin turbo straight 6 2.6l rb26dett engine. The person i bought it off had detonated the engine and the 5th piston was stuffed. The engine has been professionally rebuilt, and the previous owner has driven 500km and had it tuned on a dyno. It is currently sitting at around 240ish HP. There is noticeable lag when revving the engine, almost as if it cuts out, and even when driving at slower speeds. The previous owner told me to run a further 500km in it and then take it for another tune. If i do take it for another tune, how much extra HP could i expect to gain from it? up to 500?, and will this dramatically decrease the lag i am experiencing at the moment? It is making it difficult to drive the car, that and a heavy duty clutch is almost forcing me to be a professional race car driver to drive this thing. It is much harder to hill start etc than any normal manual car (and yes, i realize this is no normal car). Please spear me the insults at my lack of automotive knowledge and give me some useful advice of what i can expect from this car in the future. If possible also can i get some rough price estimates and the currency you are estimating in. thanks!! :)