On many cars the AIR (Air Injection Reacton in GM speak) pump send air upstream of the cat when the engine is cold to crate afterburning in the exhaust manifold to:
A) burn some of the rich startup mixure
B) warm the cat(s) to operating temperature more quickly.

The air pump is a pretty low pressure device, and i think its peak cost is bearing drag, folowed by weight of the bracket. Bearing friction might be one HP at max RPM, if it bothers you, replace it with an electric unit from a late 960 or 850.
(it seems to me the beaarings in these thigs are kind of crappy, and they might not last long if you rev over 7000 or so? anyone have experience with this?)
THe Air pump soed cary the advantage of saving you from havin to reconfigure your pulleys...On my no cat car, one of the AIR pump impellers seized 3 days after I did my conversion. I opened it up, and took the vanes out, now it is just an elaborate idler pulley. since i had no cat or air fittings in my manifolds to start with, i didn't lose anything. The system will somewhat wotk without teh vanes if teh chec valves are in good conditon....the Chrysler system works without a pump(uses exhaust pulse scavenging and check valves). You might be able to get away with adapting it to another car, but I have never tried.

KEEP the PCV SYSTEM
this emission system was developed by IHC around 1957, and trebbles the life expectancy of your engine with no harmful side efects.

The canisster purge system:
In the real world, all this does is keep your car from reeking of exhaust fumes on a hot day. 0 power loss, weight under a pound, and not active at WOT.