I can speak to a couple of these.
1) I think I only have the +2" (10" total) for cabover height and really love it. I haven't lived with the 8", but camp with many GFC folks and they constantly shuffle their bedding into their truck bed while traveling and have issues keeping it clean during the wet seasons. I really like being able to leave my comforter+sheets and other bedding or double sleeping bag above. I also can do that while leaving the soft top liner up which is great for heat/cold and quicker setups. There's been a couple rock crawly trails I've had to turn around on because of trees being a little low while navigating rocks and too dangerous to saw. Not sure if the 2" would've made a difference (I also have a roof fan). Increased cabover height also increases the height of the truck bed area when closed, this has been awesome for utility. Hauling bikes, changing in and out of ski clothing or other adventure gear, etc.
2) I have the anodized aluminum, so I sidestepped the paint question

That has really held up and has been low maintenance with our mag chloride winter roads in MT.
3) My Gladiator didn't have too many holes, but doing a tailgate seal still leaked dust at the upper part of the gate, the PPV made that 90% stop. My square cab blocks direct airflow to the PPV so YMMV. I later added a bedrug and that really blocks the tailgate and would negate sealing any holes in the bed. The bedrug really up'd the comfort level and insulation.
4) My MPG dropped about 1-2mpg, but again my Jeep is a brick. The cabover is tucked pretty close to the cab, with a Ranger I'd experiment with a fairing since your windshield is more sloped than mine. Can't speak to the RTT vs OVRLND though.