Hay mate, how did use come to the conclusion the cooler is holding it up,specially when xr6 turbos make more than this still using stock core and they are trying to cool more air.
Curious as i used to run 34 smic on my ceffy and r33 with t04 and never had problem, an they half the size.
i then done biggest mistake an fitted greedy fmic kit and it turned my rb20 into the biggest lag monster ever i mean its a joke.
Any more info you find out or dyno readings just from changing your core and nothing else would be great.
g/l with it all !