I'm a cat 4, and I race a capo. If you already had a capo, it wouldn't be a bad option. But since you have room to play around, you should steer clear. The capo is very light, basically identical to a road geometry with a shallower bb drop. i rode a friend's fuji track pro and noticed significant added stiffness. i even have record cranks and a disc/carbon wheelset. The FTP is definitely heavier but it is reinforced in the places it needs most. the capo begins to bend under pressure in sprints. it rides nice, however a well used fuji track pro is a great starter bike in that price range
Get a Planet X. Same price, but it's a carbon frame. I've ridden them and I'm just waiting for a chance to get one. I was skeptical at first because of the price, but there's nothing to worry about. A lot of people here race on them.
Fixed gear fever did a review on it and it did well