It will make a difference, but the question is how much. Standard guitar cabs have a severe drop off in frequency response below 100Hz.
Yeah, I was trying to find the response of the Mesa 4x12 cabs, but couldn't come up with it. But I see what you mean by looking at Celestion's raw speaker curves. So maybe it's not that big a deal. Maybe it'll sound a little better, but I've never thought the sound that the heavy detuners get out of their cabs was lacking. I don't know what Pete from Chevelle has in his cabs, but I love his tone.
Tough call...4 inches and 4 pounds isn't that big a deal, and the cost difference isn't terrible, relatively speaking. I keep going back and forth between the 10 and the 12. Of course, I also go back and forth on pulling the trigger on the RCF vs waiting for Atomic and Matrix to come out with their options. And I haven't ruled out auditioning the QSC/JBL/EV options from GC first yet either.