is yours a manual??
when i did my exhaust i gained about the same amount of boost so dont be worried, but the one thing i will say now is change your fuel pump as soon the stocky will run out of puff and will start to make the engine ping, what will happen is the car will make full boost but not feel to make any power.... so if this happens you will know what the problem is. but never fear, pull over, turn the car off for 2 mins then start her up again and it should stop, but dont rev the tits out of her again for a while
as for the cooler, they do fit without to much hassle, just go see a place that has experience with the cars and installs on them, coz i know it can be a prick if you dont know what your doing