You give up your paladin special mount class feature; it has no restrictions on it saying you don't get its advantages if you're doing a mounted charge.
Also, strength improves the damage linearly, just with a large multiplier - if it increased exponentially, it would multiply with itself.
Doesn't power attack exchange 2-for-1 with two-handed weapons? If so, that's +26.
PMing you the details of my paladin, btw.
Thanks for the math correction to both of you - Runestar & Elethiomel.