Last year I rode an avg of 12 hours/week from September through February as a cat 3 ('06 into '07). That includes races. No real plan other than ride as long as I could given the free time I had. Ended up with a lot of mindless zone 2/3 rides thinking time on the bike was better than not. Top 5 in local races was about it.
Got some help with structure this year and avg 11 hours/week from September through February ('07 into "08). The riding is much more focused and did interval training for the first time coming into race season. With the intensity came the need to make sure I recovered to be able to build on previous days and weeks of training. Decided to upgrade to a 2 based on solid results this year.
If you can ride the increase in hours and still recover, stay focused, you'll be an animal. If your body can't handle the increase you'll probably know it in a couple months. I realize you are looking for guys who increased the hours, but just don't forget about quality and recovery.
+1 what ZeCanon said