I can get 500+ also, but I only put on my MP setup when I'm doing something where I'm not going to be doing much (if any) debuffing. I don't really remember how much +CHR I have, but I've still managed to debuff pretty reliably even with minimal amounts of CHR.

As for merits, I put a few into MP, a few into Lullaby recast, and 4 and 4 into Wind/String skill. I'm not really close to capping my merits, and honestly I don't plan on being any time soon. I don't like exping, so I try to avoid it as often as possible. The skill+ merits are the most useful, in my opinion.