I heard back from Mark Keene at Keene Engineering. His email was as follows.
"The 1.5" jet log would be a better choice with the P180 pump. The internal orifice inside the jet log is larger and will flow more water and provide better suction. A 2" jet log would be even better; however, it will push too much water through the sluice box and affect the gold recovery. "
From this, I gather there is a diminishing return on jet log sizing, and 2" is likely the max doable for a 2-1/2" dredge. I also take from this that the larger the jet log (provided adequate pressure is maintained), the better the suction. I'll likely purchase the 1.5" power jet for now, but I'll look at having a custom one fabricated for larger in the future.