Celica st205 JDM model gutted cat HELP, gutting the cat yes or no? Options

[empite]

Gutting the stock cat is there a problem? not about the tests or whatever.. about the engine... is there a problem that the cat stock pipe is wider than 3 inches..will cause any damage to the internals because of no out gas compression ? i gut the stock cat almost 3 months ago and now i have some problems misfiring on high load , O2 sensor running rich like always , changed spark plugs and igniter + MAF the problem is softer now before was like fuel cat. hardly cuts now its like misfiring in row on full load of stock turbo pressure...and there's a friend that keeps telling me that the gutted cat does this.. is this possible after 3 months of gutted cat starts a problem from nowhere and causes this ?

Thanks in advance