In the case of the Geo 1 liter engine a little less exhost restriction helps a lot - I saw a bin increase in acceleration in 5th gear when the resonator is bypassed - never played with the cat other than replacing it once. In all cases the less restriction in the exhost in that little engine the better it will run because less pressure on the piston when punmping out the exhost stroke the better it will run and there is not too much need to worry about extra scavenging or exhost charge back flow etc in such a small engine. As far as the valves burning out . . . not going to happen in this small an engine with such small valves . . . if anything it will keep them cleaner by blasting off any deposits which I doubt with have any effect is you are striving for high FE you are not pushing the RPM and power levels high enough to burn valves.
Well, you guys told me not to a while back, but I put a header with small primary tubes (chikara torque step) on my civic hx to experiement whether or not backpressure hurts FE. I expected that relieving the engine of the power negating effects of backpressure would help FE..... I can't say I was wrong yet, but my fuel efficiency went down from always around 42mpg to 37-38 mpg even under the same driving habits and weekly 120 mile highway trip. The main reason I expect is the relocation of the 5-wire O2 sensor from a cast iron exhaust mainfold right out of the engine to a steel o2 bung beneath the engine. I plan on relocating the O2 sensor with a weld in bung to closer out of the engine were 2 cylinder tubes come together and wrap those two tubes up to that point in that fiberglass heat wrap that someone tried to use to keep their pipe cooler. Hopefully that will keep the O2 sensor hot enough and I can more scientifically tell the difference in FE of less back pressure.
the guy said his cat disapeared, not his hole exaust, he could have very easly said "my whole exaust system from the cat on back disapeared" but insted he just said that he replaced the cat with a straight pipe... so to me it sounds like he still has a muffler, and alot of the correct sized piping, so with that said, I would say he still has decent back presure, and that the exaust is still tuned pretty close to right, but he is lacking an old honnycomb catolitic converter with all those 1/16" holes to push exaust gasses thru.
with that said, I don't like smog, and do like clean burning engines (cvcc)