Yeah, there's a range of test data out there for both the EJ25 engine and about exhaust in general including specifically cat flow and resulting HP differences.
Without digging around for a while, the gist is the stock cat fits the stock engine well enough. A high flow cat is only a minimal improvement and still requires a full exhaust upgrade and good header if you cared at all about power. The difference between a high flow cat and no cat is negligible. It's not even that much of a difference with a lower flowing cat. Realize that the limitation is mainly on the top end, and our engines are already rolling off in power at that point. You don't really worry about flow until the engine is first capable of pumping that much in the first place, the whole engine: intake, throttle body, intake manifold, heads, cams, header, exhaust pipe, and muffler along with the cat you're so worried about.