I fit into the low-end of the Cat4 range apparently at around 275-285W (on a good day) for 88kg . Heh, nice to know. If I get down to where my weight should be (purely subjective), i.e. around 75kg, I should be able to scratch the bottom of Cat 3.
My "field test" is riding-up "l'Alpine d'Huez" about 9 miles north of the GWB. It's only 2km long if you start from the river level but it's a good test at 7% grade, using a HRM to stay within my limit (Lance's test portrayed in I thank "Lance Armstrong's War" was a 1km hill outside of Girnoa at 10%).
BTW, speaking of Lance in the Alpe d'Huez TT, I've never seen determination as I did on his face as he passed by us. O - M - G. Now I know why... 516W for over half an hour? That's sick!
Edit: clarification