OK, first off 7.9-8.0 is not all that low, especially for a FO tank. What lighting do you have? If you will be upgrading when you go to reef, the more intense lighting will help to raise O2 by increasing photosynthetic activity in your tank--and increased O2 will also raise pH.
I wouldn't recommend CC in the sump. It will become a detritus trap and might actually lower your pH. If you want to put anything in the sump, consider fine oolitic sand and some LR. The CC would be a pain to keep clean and IMO would cause more problems than you're trying to solve.
I'd not worry too much about the pH at that level. You might add sand and LR to the sump for stability.
Is your alk number in dKH or meq/L? If it's dKH, then it's low and you need to buffer it up, which will help to stabilize your pH. If it's meq/L, it's actually a little high and that wouldn't be the cause of the lower pH.
My recommendation would be to go forward with your reef plans. If you will be upgrading your lighting, that will help to elevate your pH. You might consider using part of the sump as a refugium to grow macroalgae on a reverse or continuous lighting system. That will also stabilize your pH. At any rate, I wouldn't sweat your pH too much at this point, as my belief is that once you "reef-ize" your tank, it will come up on its own.
