[MiracleMax]

well 1-5/8 are probably the better of the two choices. Although it depends??

Are you talking lightly modified (as in just bolt ons, including no heads?) If yes, I'd say even maybe 1-1/2 would do a good job (or 1-1/2 to 1-5/8 stepped headers)

1-1/2 shorties, with a correctly sized X-pipe (read proper dia to work with exhaust flange on header) would do a good job.

The trick here is to maintain exhaust velocity through out the system, yet elminate back pressure.

Ideally zero back pressure is where you want to be so that there isn't any pressure acting against the piston, sapping HP.

There is no plausible reason for maintaining back pressure in the system since it does nothing except hurt the output of the engine.