I'd say you're on the right track- put in a better cat. The OEM one is big, and has two sections of catalyst, basically two cats in one.

And don't fall for the "high flow cat" sales pitch. They don't even make the old "low flow" style any more. They are all high flow.

If you are in CA, new laws require you to use a CA approved OBDII certified cat, and the smog tech will check the numbers stamped on it (if he checks and notices its not OEM).

'97 4Runner, '06 F350, '86 4Runner, '05 WR450
http://home.4x4wire.com/erik