Could putting on a full 3-inch exhaust system with a high-flow cat, in any way decrease performance in my 84 Z28 HO?

I only ask this cause ive heard roomers that too much exhaust flow is bad, and that if there is too much, some valve can't open any you will fail emissions. Is this true?

Would I get better perfromance by converting it 3inches all around with the highflow cat, or 3inches with the highflow cat going into the stock 2.75inch pipes?

A single 3" system will not hurt performance.

The valve you mention is the EGR (exhaust gas recirculation valve). I have yet to get an EGR code displayed on my camaro, but it would set a code on my chevy truck when I added headers, removed the cat and ran short pipes. As long as you keep your cat, I would think things would be ok.

I agree, you will only see a performance increase with a single 3" exhaust. A dual 3" with no cats would probally remove all backpressure, but if your'e running a hi flo cat or dual cats you'll be fine! My cats "fell off" after I put my exhaust together and I saw(or felt) a significant gain, with no problems regarding the egr.

So what about a 350TBI running a 2.5" dual setup?
I was thinking about the Pacesetter or Edelbrock shorty headers that are 1 5/8" primaries to 2.5" collectors. Then using a 3" crossflow like a Flowmaster 80 with dual 2.5" tail-pipes.

The "issues" really don't come up unless the header primary size is too large - such as using 1-3/4" primaries on 305 TBI swirl-port heads.

1-5/8" primaries aren't going to be a problem for a 305 or 350.

thanks alot, i was worried there for a minute

Actually the stock manifolds are so crappy even 1 3/4 headers will make an improvement. I think a member here dynoed before and after with SLP 1 3/4 headers an an L03 and picked up 6 rwhp, and he already had a custom y pipe and exahast all the way back before. I still agree 1 5/8 would be better though and make more power at a more usable rpm too.