Well I put the 35mm G-Ones on last week and the guards fit just fine. There is a few mm clearance each side where the stays attach, so they seem fine for 35s. You do need a fair bit of fine tuning to get them central. I guess 37 probably would be a practical max. I'm liking these Allround tyres, they are giving me a bit more confidence on wet mucky roads.
Went on a nice 10 mile ride yesterday with about 3 miles on little farm roads. These have tarmac but lots of runoff water, and a sludgy covering of mud and horsemuck in places. I'd planned 15-20 but it started to hammer down about 6 miles in.
I can't say I have noticed any difference in speed or acceleration yet.