How do you do on the fast group rides in Colorado? (the ones with racers driving the pace)

You really don't need much power to hang in a race that doesn't involve hills. When I "gave up" racing, I was showing up every two years in a 4/5 race and could get top 10 every time, 50 lbs over weight with poor conditioning.

You should just sign up and go. Here in TX, only about 10% of the races seem to offer pure Cat 5 races anyway. Most have a 4/5 field, so it's about the same as just having 4s.

The problem you'll have is that the pack surges all the time, so you have to be able to ramp up, then recover quickly. They'll chase down every break almost immediately, which causes the surges.