2 OS 2x12s sound much bigger than a 4x12. I've had 2 2x12 Mesas on top of each other too, still they sound more directional than a 4x12.
You can carry a Bogner OS 2x12 by yourself, but I doubt you could do it with a 4x12. So there IS a difference.
My Bogner OS 2x12 sounds much better to me than Mesa 4x12 does anyway.
To keep it simple, I have had many cabs & they are all gone. All I have are the 2 Bogner OS 2x12s right now. I'd like to have an Ubercab, but there is not much difference really.
I liked the Bogner 2x12, but i wasn't as impressed as a lot of other people with it. It's soo big you may as well just use a 4x12, and it costs more than a lot of 4x12's.
No kidding an OS 2x12 is almost just as bulky as a standard sized 4x12, so I would just go that route personally. One of the big reasons I got a couple 2x12's in my cab collection were for their compact dimensions specifically, to make it easier to transport them for jams and stuff. Otherwise I'll go 4x12...
Can't speak for the SigX, but the Mark IV sounds great through it imo.
I actually took the V30's out of mine awhile back and dropped in 2 WGS British Leads. Sounds even better with the amp I'm currently running through it, less buzzy and smoothed out now since the WGS BL has a flatter more neutral voicing.