you have to consider the whole system and the REASON backpressure is a concern (it's not).
most people talk about backpressure like it provides something to the equation. well, again, we have to consider the SYSTEM. scavenging depends upon VELOCITY in the exhaust gas. the velocity is what creates vacuum at the valve outlet. if the pipes get too big early in the system, velocity stalls and pressure increases. this means no scavenging. we also have to consider that as the exhaust gasses cool, they condense a bit, leading to further loss of velocity (this is why people try to keep the heat in headers).
ideally, you'll have a properly designed header. the length & diameter of the primaries will be tuned to the engine's parameters (valve timing, duration, diameter, angle) and to the desired performance charictaristics of the user (high-band power, low-band torque). they will join into a merge collector that is a venturi. it will neck down slightly & then open up a bit (like a megaphone). the header itself will accelerate the air past the venturi (secondary flange). once this happens, you want LARGE pipes; back pressure is not a concern.
if you're using non-venturi style headers or the OEM manifold, it's not the loss of cats & backpressure that causes the problem. it's velocity loss due to the DIAMETER of the tube. the bypass pipes are all 2", but the OEM pipes are SMALL. this means that you'll lose velocity & scavenging lower in the RPM range. there may be some improvement at the high end. all else equal, removing the cats will greatly improve velocity & in turn, scavenging.
with OEM exhaust manifolds, 2" bypass pipes and the pedro muffler, you will probably lose power overall w/ the 2.5L and 2.7L motors. there MAY be some high end power gains. the 3.2L and 3.4L would probably make power.
if you go w/ decent headers on the 2.5L / 2.7L, the bypass pipes & pedro muffler will make power up high, but lose some torque. i found the 2.5L to be gutless w/ torque anyway, so i always opted for high RPM power & revved the crap out of it. much more fun for me!
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