I think there are plenty of people in my area that can test FTP at 300W (meaning about 315W for 20 min), but few that can actually hold 300W for an hour, or at least few that have done so. I'm one of those that tests at 300W, but I've never come close to holding 300W for an hour. My numbers are very similar to Tmonk - my PR for 20 min is 325W, my current 20 min test is at 311W (both outdoors). The difference is that I'm 25 lbs heavier than Tmonk
. There are a lot of people here stronging than I am, so maybe many that can hold 300W for an hour, I don't know. But there probably aren't that many that have actually done it, and they are likely mostly 1s or 2s.
I disagree with the statement in the original post that if you could hold 300W for 45 min you could ride away from a Cat 4/5 road race field. Maybe you could if you were light and the road was going uphill. Or you could on flat ground if you first put out 700-1000 watts for 10 seconds to get a gap, held 450-550 watts for a minute to grow it a bit, then settled into 300W. This is quite different from doing a stead 300W for 45 minutes. And even so, most 4/5 fields around here will work together and drafting is significant enough that they would pull someone back doing 300W.