2) What's over powered about it? intangibility is a moot point in the NW, and more fire-power (pun not intended) is exactly what he's gonna need if he ever needs any kind of power-up.
3) You're underestimating Oda's creativity if you think the Mera Mera is gonna change his fighting style in an unpleasant way. Seriously, just look at Red Hawk, is that what you'd call umpleasant?
4) Luffy doesn't brag. You ought to know that by now.