1. I'm guessing--but--I bet the '91 still used a pellet-style converter rather than a "honeycomb" "monolithic" style. There's a stainless-steel mesh thats supposed to keep the pellets in their proper place in the converter; but the mesh tears or rusts, and the pellets migrate "downstream" due to exhaust gas flow. We used to smack the muffler of every GM vehicle that went up on the hoist, listening for the Maraca-like rattle of a gazillion little beads bouncing around inside. And it was not at all uncommon for vehicles with pellet-style converters to spit one or two out of the tailpipe as they're fast-idling in the driveway, or at stop signs. Surely they spit them out going down the road...but who would be able to see them?
2. The EGR valve on the TBI engines is a "negative back-pressure sensing" style. Changing the OEM back-pressure affects the EGR valve operation, and therefore engine driveability and emissions compliance. I have the sense that it's not a critical deal; just be aware that back-pressure is deliberately tuned on the TBIs. However, for PERFORMANCE use, "zero" back-pressure is better than "some" (any) back-pressure. If a performance engine runs better with backpressure...it's not tuned properly.
3. When it was me, I put a honeycomb/monolithic style aftermarket converter on my '88 K1500. Nobody likes stinky exhaust, we all need to breathe clean air, they're not that expensive, and they don't kill performance ESPECIALLY at the low-to-medium RPM range typically used by a pickup with shiitty cylinder-head flow like TBI swirl-ports.