One has to be able and willing to delve into some involved physics to make definitive predictions that might result from (1-7/8" vs. 2") primary cross sections: length, area, rpm, gas volume, resonance, etc., etc. And, because the cross sectional area difference is relatively small (less than 14% for the smaller tube),
length will become more important than area alone in the final answer
(in this case).
All things being equal, the smaller diameter primary will favor slightly lower RPM than the 2",
until the inherent back pressure (of the smaller tubes) "pinches off" further gains to where the 2" tubes become superior.
Length being equal, side by side comparisons would ultimately be needed to distinguish the practical difference between them. But, from this frustrated physicist's perspective, I'm betting a very slight superiority in peak torque at around 5200 rpm for the smaller tubes (perhaps), but torque "under the curve" above the peak will favor the slightly bigger diameter tube.
Sure would be interesting to see, huh?
Bench racing... Gotta love it!!