cheers for the responses guys.
Cooler is a 600x305x115 and I already purchased it.. was a bit of an impulse buy but oh well... my old 75mm thick, 2 and a half inlet/outlet ARE cooler was starting to reach it's limits at around 23-24 pound - im guessing due to the inlet/outlet size and but mostly due to a quite a few crushed fins from retarded tow truck drivers over the years. As long as the difference in lag is minimal and I can run more boost I will be happy. I will eventually upgrade to a 42r anyway which will probably be more suited to the cooler.
In reference to R31Nismoid's comment, the car currently has a 2speed glide behind it.
I guess all I can do is see when fullboost is with my current cooler and then see what the difference is with the new one and keep you guys posted!