[Plums]

Under-body vs Sidepipes? There is a discussion all of it's own. Basically, sidepipes are short and usually large in diameter, this combination is not good for low-down torque, and in many instances can hurt the top-end too. The lack of a cross-over hurts too.

As usual, Mike has hit the nail on the head with his cross-over theory. For the average Commodore running an LS1, a well designed cross-over system will win every time. Perhaps not for power right up at peak RPM (some other designs may win out there), but average power under the curve the cross-over definitely wins.

As for this "pipe size" discussion, I don't know that any of us will ever agree.
One thing I discovered during R&D sessions with Pacemaker, was that LS1's generally don't conform to normal header pipe theory.
Pipes that we thought would work well, were often outgunned by pipes normally reserved for the race track. Some of the reasons for this I'll have to take to the grave with me, as they are commercial-in-confidence.
Here is something to consider though: Many years ago, people were putting 1 5/8 pipes on 5 litre Holden engines (that was considered the norm). These engines produced 165 kW, or 185kW later on. The LS1 power has ranged from 220 kW up to 260kW.

For Pneall, I am interested in your case. What size pipes did you start with before you went to the 1 3/4 pipes? Did you just go up in primary pipe size and everything else remained the same ie. sidepipe size, collector size etc.??

Cobra.au, with the power you're making I think you've made a wise choice...