Batts of any type have inevitable installation error issues that impact performance. Compressions & gaps allow air to convect around batts with very little impedance to flow. Low density batts (like R19s & R11s) have high convection rates at the temperature extremes even within the batts, and do very little to slow down stack-effect or wind-induced infiltration, as Jim points out. The loss of performance can be significant, particularly in homes that were built without air-tight detailing at the sheathing &/or gypsum. In attic applications without top-side air barriers the effective R of low density batts at 0F outdoor temps loses about a third or more to convection, and it has the summertime radiated heat translucency problem cutting into performance too. High density "cathedral ceiling" batts do MUCH better, but only if installed perfectly for a perfect fit.
Blown or sprayed insulation fills all gaps, and cellulose at ANY density is an order of magnitude more air-retardent than low density fiberglass. At "dense pack" density (3lbs+ per cubic foot) it's two orders of magnitude more air retardent. In an open blow attic situation it takes at least 3" of cellulose overtopping, but it will in fact restore the effective-R of the fiberglass to something close to it's tested values when you do, simply by the air-retardency of the cellulose blocking the convection loops between the open-air of the attic and the fiberglass.
In wall cavities blown new-school fiberglass at 1.8bls or higher density can be as air-retardent as 3-3.5lbs cellulose, but it's usually more expensive. At 1.0lbs density it's no better than R13 batts, whereas low density (~1.8-2.2lbs) cellulose is still pretty good, but either would settle in wall cavities over time in NJ climate, where dense-pack would not.
Cellulose with "borate-only, sulfate-free" fire retardents doesn't stink when wet, and doesn't cost appreciably more than products that use sulfate fire retardents. The sulfates are cheaper than borates, but are corrosive to metals (particularly copper, but iron too) when wet, and banned in some countries. Cellulose designed for wet-spray (sometimes referred to as "stabilized" cellulose) contains water-activated adhesives, but no sulfates, and it's fine to dry-blow them rather than wet spray. For DIYers using box-store outlets it's sometimes hard to find the borate-only goods, but it can be special ordered even through box stores (but it may not have the same liberal return policy give to the off-the-shelf stocked items.) It's not a disaster to use sulfated goods, just be sure to remove it quickly if it ever gets wet from roof leaks/ice-dams, etc., and your nose will tell you when that happens. I have several places in my house insulated with sulfated goods, but it's never gotten wet, either from bulk moisture or condensation. YMMV
FWIW: The worst application of suflated cellulose story I've heard never should have happened: Cellulose was installed in an historic building built with clapboards directly on the studs (no sheathing, not even planks), guaranteeing that wind-driven moisture would keep the outer layer of the cellulose damp, if not soaked. The 200+ year old nails all began to rust through, the place became structurally unstable and began to sag, and the place had to be condemned. The same might have happened even if borate-only goods or fiberglass was used, but probably would have take years rather than mere months, and there may have been time to save the building. Almost all professional cellulose installers know better than to do that.
Thanks! Most helpful. Any insight on my plenum question?
If the plenum is undersized and restricting flow it's an issue, but how big that issue is relative to the rest of it depends. It won't fix the balance issue between the two end rooms, but it can only help the overall air-flow. Without sufficient information on the whole duct system it's hard to say how critical that is. Duct system issues aren't always well suited to re-design-by-web-forum beyond the more obvious stuff.
If you've already FSK-taped &/or mastic-sealed every seam & joint (as well as on the airhandlers), and caulked the duct-boot to the floors/walls/ceilngs at the registers that's about the most you can do without re-sizing ducts where they're restrictive.