So I've had a total of 3 HVAC contractors take a look. Long story short, the 2nd contractor to take a look discovered that the manual damper was incorrectly installed (by the initial duct contractor). The manual damper was showing open but was in fact.....CLOSED. No one thought to check it since they assumed it was installed correctly. Once I opened the manual damper, the static pressure dropped to 0.85" w.c., which is still a bit high but MUCH better than 1.3"! I feel so stupid. The vents are now flowing decently and distributing heat.
So where I'm at now is this:
Two contractors said the trunk is still a bit small, but doesn't seem to be a deal breaker since it's distributing heat, albeit still with some whistling, which is pretty annoying. But the blower doesn't sound strained like it was before. Also, in anticipation of doing the trunkwork, I exposed the trunk by ripping out all of the tongue/groove board. I asked if I should do anything while I have it all exposed. Aside from a few minor things and some efficiency upgrades, both contractors recommended to wait until I do the addition so that I would know exactly what the final demands of the house are.
But I keep going back to my trunkline only being 128 SI, and feeling like I should increase it. Then again, that could be revised once the addition comes in 2-3yrs or so. Decisions, decisions.