I've tested Scopebox for months, since NAB 2011 - experienced the same exact problem. Very low performance (~3fps), high lag, and instability, on G5 with Kona2 card. Practically the same experience with v2 and v3.
Divergent Media are aware of it but I'm not aware of any fixes. The last I've heard was that they were "tracking down performance issues with PPC... I'm working on a few optimizations and additional debug options so we can have users record additional info on their systems."
Personally I gave up and moved on. Scopebox is still a great system, it just needs an Intel Mac. Get a mini... or just get a Blackmagic Ultrascope with a cheap PC.
Hard to say, I talked to the developer a bit about G5 performance and he said it is a combination of CPU and GPU that makes SB run. I suppose that a better GPU than a stock 128mb card would boost performance but it is hard to say if 1080p video will work even in 8-Bit.
We have been working on trying to boost performance for all machines. At this point, it only makes sense to make changes that speed up both PPC and Intel... PPC is just too old to devote a lot of resources to going forward (and apple makes it harder each year to even ship an app that runs PPC and Intel). That said, we found a solution to a slow down some users have seen with slow GPUs (this includes many PPC machines).
If you'd like to give the fix a try, send me an email at mike [at] divergentmedia [dot com] and I'll set you up with an internal build of the changes.