Hi guys, the standard exhaust is just about falling to bits on my 1993 zx16v phase 1.
I was thinking of maybe a cat by-pass tube and a straight thru center section? or a conbination of these conected to the standard back box.
what do you guys think i should do? how much more power will i get with just striaght thru centre section? with or without cat?
Please give some help....

It should release a few horses yes. BUT I would suggest to take a look for a better rear box too sooner or later.. It will be a bottleneck, although I thought that the std box not too restrictive is.

The original exhaust system is actually quite good. There will be some improvement but the gain is small.

I' ve made the described conversion with some slight difference. The entrance of the catalyseur is with 63 mm. diameter, and its exit is 55, or 50 - I don't remember any more. I' ve put aproximately 1.3 m. pipe with 63 mm. diameter instwead of the catalyseur, then it goes on 55 sm and reaches the standard Walker muffler (which really is quite good... and heavy). The result is satisfactory. I recommend it!