We have 2 F-450s with 17" rims and they don't ride that bad at all.
But their not loaded down as much as you are either.
I don't know how the rim size would affect load rating tho. If it did at all.
Parallel turbos are more for top end, little harder to spool but can move a lot of air once they are lit. Common in gas engines because they run smaller turbos, then gate them on the top end, and have a lot of air flow over a wide rpm range.
I'm at 4" as well, just thought that having the high steer would eliminate the tie rod rolling and give better steering
Yeah got the pitman arm dropped