Wanted to get it done anyway so I just went ahead and installed the V30s in the bottom row. Soldered the connections, used the screws that were still usable to mount the back and gave it a quick testrun.
I can't crank the amp today anymore but my first impression on low to medium volume was that the cab gained some grit, sounds fuller yet tighter. I looped myself with a Boss RC-3 and held my ears close to every speaker, odly enough the loudspeakers don't sound that far apart. The G12-65s have a bit more warmth and bass, the V30 have a bit more grit and aggression as expected. However I can instantly tell that they are broken in as they sound a lot more pleasing and not as raspy and spikey like all the new V30 cabs I played in local music stores.
I'll try to report back with more findings as I spend more time with the cab, will probably do some micing shootouts and see how the character changes with higher volume levels, but for now I'm happy it all worked out. The 2 spare 65s will be stored away in my closet to serve as a replacement should one of the 2 I have in the cab ever go bad. Great speakers, if you can find some original ones from the 80ies like mine are I highly recommend them!
On another note, what are some common and good mods for those Marshall cabs? I heard of replacing the plastic handles with metal ones, sealing the gaps, using some heavy duty wire and tightening the back to the post, anything I missed? And is there a site where I can look up the mods and have material I need listed? First thing I'll do of course is go to my local supply store and get some new screws.
Cheers!