Less light, less growth. Not a 50% reduction in growth though, as your plants weren't able to use that much light anyway.
Try it and see. Use whichever bulb looks acceptable on its own, which will probably be the 6,500K. Make sure you still get a decent light distribution, if it's too uneven from front to back, you may find going back to the CFLs preferable.
Fuzz is a bit resistant to H2O2 in my experience, but it can be killed with it.
Great! Yeah I highly doubt it'll be a 50% reduction in growth, I was getting decent growth with the 20W CFLs and most of the light was escaping out of the sides. Should I reduce my EI Dosings as well? I'm dosing for a 10-20g.