The girl and I have the Catalyst covers. They're mostly for bling (the difference between them at the Wheelbuilder ones, that is), but they are pretty, and very light.

You don't want tubular anyway. Make sure your rear wheel has a good amount of surface area below the brake track for taping to, get the cover made without fasteners, and tape it down with 3M Super33+ electrical tape. And I've never seen anyone demonstrate a difference between a "true disc" and a cover. Some of the "true discs" (including Hed) are just permanent covers over spoked wheels anyway. The air doesn't care what's on the inside.

Get the cover. Get a good aero helmet. Get a tight-fitting tri suit. Get yourself fit by a Retul fitter at a tri shop. No reason to skip any of those steps.\

Heck, my cover lives on my rear wheel for four months out of the year. Too lazy to keep swapping it.