I'll be mounting a circular/oval shower curtain above the tub.
OK got it! Kinda assumed that was the intent.
That being the case you don't need the Permabase. To use it would be a waste of money and create seam issues you don't need.
I would use MR (moisture resistant) Drywall. That's all that would be required. No moisture barrier would be required either. Tape and bed the drywall seams and install the tile. In this case the tile could even be installed with mastic if the tiles aren't going to be too big.
The floor may be a different story however.
How is the floor constructed?
Joist size, joist spacing, subfloor material over the joists?