I'm using two 40w's, but if I hadn't gotten the divider and just used one 80w, Yes, I would install it off-center. I'd still try to keep a few inches between the edge of the rhp and the side of the enclosure if possible. If I just use RHP heat, I can have one side with these temps:
Warm side - 91/92F inside the hide, 88-90 surface temperature
Cool side - 78F surface/ambient and about 81F inside the hide.
These can be adjusted, and live in a 100+ year old house, so it can fluctuate heat a good bit. The temps in my T8 generally may fluctuate a degree or two, but that's about it. I have one Herpstat probe inside the enclosure for my RHP, I have two accurites (one warm side, and one cool side) and I also use a heat gun to spot check my temps a couple of times a week.