Cat bypass with o2
 

1987 02 sensor frozen no need for cat, what can I expect,installing Brush bypass

will probably run rich.

Expect a little less back pressure. A touch more favorable rumble. Maybe a few ponies.

Make absolutely sure flanges are sealed upstream of the sensor. I have used two factory gaskets with $8 muffler paste from the local store.

a lot more stink at start up. i liked the sound better with the cat bypass. i thought i could feel a little more oomph, but now having gone back to the cat for smog, i'm thinking no noticeable difference...

A bit more noise and a bit more smoke/fumes on start up. You might get a few HP but only at WOT...most confuse the noise for HP.
If you notice a big difference it's because your cat has partially failed and is clogged.
You will lose a big hot spot next to your cam cover tho.