Shred: I don't find the fizz to be too much of a problem, as long as the amp has its settings complimenting the speaker you can make it work. Obviously you will want to LP anything above 10-11k, but don't be afraid to move that mic anywhere if it works.
Yep stock G12T75 speakers, the more I use them the more I love them. They have a different sound to V30's but are definitely usable. I had an Orange cab in here the other week, and I was actually a little disappointed with it, compared with these Marshalls, which I found surprising considering what you hear about them. That said I can see it working for other projects. Right now I have my friends Triple rectifier rig and its very tasty - think its staying in here for a while!
With the setup I use, I know for a fact its far from ideal but I really do know these speakers inside out, and are far from the worst things I have heard. I am planning on picking up some A7's though.....need a few more payments first though!
I think room acoustics are important, but I don't hear too many problems with this room. My room at university for example is quite a bit smaller and I can hear some pretty nasty reflections in there, untill I put my huge flag on the wall anyway. I have noticed my ears are really responsive to reflections and I notice them in all sorts of weird places. My friends think its so weird haha.