#1
I was looking at my MAG earlier and saw it looks like I can unscrew the head from the combo box. Not sure if the screws will actually remove it but if I removed it could I just hook it up with a 4ohm cab (not sure which, cheap yet good options would be good) and use it to its full power rather than have a huge two cab rig? Or would it be cheaper/easier to have the combo and extra cab?
Gear:

Musicman SUB 5
Ibanez SRX305
Ibanez GSR200
Ashdown MAG300 C115
Ashdown MAG 210 cab

Soon a Fender Deluxe Jazz V & an LTD 206
#2
combo + extra cab for pure giant volume goodness...

But... it looks like you've already got the 115 combo and a 2x10, I would suggest keeping it that way. 2x10 (in my experience) sounds a little thing compared to 4x10 or a 15... but just a 15 usually sounds too dull, so, having them both will help your sound out.
Fact: Bears eat beats. Bears beats Battlestar Galactica.
#3
I have actually done this.

It works wonders when You go to band practice.

I still just use it with the extra Cab.

For gigs I'll take my 1x15, and the 4x10.

for band just the 4x10.
"Whats that noise??"

"... Jazz"