91 formula 5.0 TPI original exhaust except a Flowmaster muffler. If I remove the cat and install a pass through pipe will that effect the performance? Keeping the o2 sensor in closed/open loop? And will it generate a check engine light? Thanks

 

It's OBD-1 and the O2 sensor is upstream of the CAT, so there won't be any Check Engine light.
There's no real performance benefit to removing it unless it is clogged up or damaged, in which case the right thing to do would be to replace it...

 

Old school catalytic converters like these cars came with were power robbers. That's not the case any more. You can replace it with a modern cat and it will work fantastic. Different internal construction resulting in different exhaust flow characteristics.

 

Your 91 TPI came with what's essentially a "modern" catalytic converter, for all practical purposes. You can quite literally read a newspaper through it.

So, no, you'll see little if any benefit from replacing it with a "test tube", unless it's defective in some way (broke up into chunks and blocked for example, or shedding chunks into the muffler ... yes I've had both happen). And even then, the difference between a tube and a non-defective "modern" cat, is undetectable. Especially on a 305.

Bottom line: if your cat (the part, not the animal) is farkled, consider a "tube". If it's not, leave it alone. There's no there there.