Maxing out higher is irrelevant, 5 CP doesn't mean anything.
If you want a future-proof Groudon - i.e. you're worried that the one that has 13 attack IV won't be able to hit a breakpoint - then max out the one with 15 attack.
A difference of 3 defence and 1 stamina IV is not significant, especially if compared to the potential difference of hitting an otherwise unreachable breakpoint.
Regardless if you're using a maxed out Groudon to clear a raid that it is useful in, you're going to clear it with ease even if you are missing out on 1 damage from your quick move so I wouldn't worry about it too much.