I'm a few states north of Florida where the question was asked from. In the spring and fall I have no problems shooting a .678 underbore - and I would guess that paint generally runs between .682 and .686. In the summer, it seems like the .678 will only work in the mornings and never in the afternoons. By mid-day, the paint will have moved straight on from hook shots to exploding in the barrel or jamming up at the bolt. On summer days I switch back to the .684 or the stock bore .689. I've never had a problem with the .689.
I would imagine a similar situation in Florida, only a longer cycle of needing a larger bore for paint swelling. The tradeoff is that he should be able to underbore in "winter" when it's only 60 degrees outside.