The main issue with the aftermarket cats are their size. Oem cats are easily twice the size of most of the aftermarket ones, and is why they seem to have an issue. Side by side comparison.
[Linked Image] [Linked Image]
The "X" indicates the area of the combustion chamber, screen to screen.
Given the new CA smog regulations, the CARB approved cats shouldn't have the same issues as the old ones did.

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