I've got 3000 hours in a Metro III but I haven't flown one for four years--so I apologize for my incomplete answers.
I would *rarely* operate the Metro at FL250 (maybe wintertime) but it seems like an IAS of 120 would be pretty low.
I remember flight planning for a TAS of 260.
I can't help with the other performance numbers because my books are packed away.
A small fuel inbalance was pretty typical and not a big deal. There's a "cross-flow" valve between the two tanks that's operated by an electric switch. By opening the valve you can allow fuel to either gravity feed the other tank (on the ground while parked on a slope) or you can fly in a slip and let the fuel gravity feed in that manner.
Alternatively (and the fastest I found) is to open the valve and then taxi in tight circles while allowing the fuel to cross-flow due to centrifugal force! But I don't recommend that with passengers!
