W/o the cat you would have no increase in power in the area powerband most of us use w/the 3.0, you might get some power above 4500(not really a usable rpm range).

If you are looking for a better flowing exhaust, pick up a new cat at Autozone for $55(universal) or $75(bolt-on).

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed