If the tire diameter did change, it would error in the way of better numbers I believe (that's if it showed up at all).
I'm still new to the six cylinder import cars, but I've worked with American V8's all my life. Every time we've opened up the exhaust to a free flowing setup without changing anything else, we've lost torque and gained horsepower. The exception being, the exhaust starting out really really restrictive to begin with, in which case we gained both hp and torque.
As a rule of thumb, shorty headers were better than stock manifolds and would help keep the torque down low and increase hp up high. Full length headers would give you even more hp up high but would sacrific torque down low. Removing the cats usually resulted in the same as going to FLH. FLH's really start to come alive when you start going with a more agressive cam or forced induction. Primary size is usually the most important factor in any header. Too small and you're not able to flow enough air. Too big and you loose all your velocity and ability to scavange.
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