there are a lot of variables to consider, such as other engine mods, but, assuming the cat is the most restrictive part of the exhaust (many monolithics are not as bad as the old pellet type), removing it will most likely net hi-rpm hp gains because of improved flow and higher air velocity. this is often at the expense of low-end torque, because exhaust scavenging is hurt. some backpressure is needed to reflect the lower-pressure pulses traveling through the pipes and manifold to aid in removal of exhaust gasses from the chambers. pipe bends, surface texture, temperature, etc. create a very dynamic environment in the exhaust system. crunching some numbers with pvnrt (PV=nRT) can explain and quantify some of the mess.
my k5's old cat was gutted, and when i replaced it for inspection, my low-end grunt improved just a little. on my other truck, when i got higher-flow exhaust, my top speed improved significantly, but my off-the-line suffered a tad. that has been remedied, though.
