I put an OEM 7900 P&C on my Makita dcs6401. I was very impressed with the quality! I have an 046BB (newer casting), and it seems to be pretty decent quality, but that OEM 7900 jug is nicer than any oem stihl, husky or anything else I've seen.
I paid $189 for the P&C through Baileys (love dealing with them). That's a $70 difference between the BB. I figured I'd rather have the peace of mind and the good quality of the OEM part. I think the BB kit could last if you put good rings in, cleaned up the casting a bit, and ran good mix. I just trust that OEM jug a whole lot more though.
I think Baileys would do real good by selling a package deal Makita 6401 and OEM 7900 P&C for about $670. There may be an issue with Dolmar not allowing that though.
As far as the carb- if you back the H and L way out then it'll just be super rich. Carbs just control the ratio of air:fuel, and if you want to put more fuel in, you'd also need to have a bigger bored carb to catch up with the right amoung of air. As far as I know, the 6401 and 7900 have identical carbs anyway- and have the same factory carb settings (L=1, H= 1 1/8)