Honestly I think that the WA and the 2 12s would get it done. I own that stack in 15" and it gets STUPID loud. Those cabs are just really efficient. I have done quite a few pretty loud gigs with just the combo and no extension. I usually throw both in the car and just set up one at sound check. If I feel like I need more boom I go to the truck and get the other one. But a lot of times I don't need it. And the tone is AMAZING.
(Although full disclosure I also on a vintage V4 and it also sounds really REALLY good. I LOVE being able to say "Which really awesome sounding rig am I going to take out tonight?) Anyway.....
Is there any way you could take your current rig to a place that carries the Walkabout stack and A/B them? I think you will be surprised. I have always likes the sound of those Ampeg 1-15s but they don't seem to pack a huge punch volume wise.
Of course, Jimmy makes a good point too. If you like the tone of the V4, just get the new head and one good, efficient, loud cab. You'll still be in heaven tone wise.
The good news is that the tone of either is great. You can't lose there.