I imagine that you couldn't put on an aero helmet and not ride any faster, you'd feel like a fool so I'm not totally skeptical. I'll bet a skinsuit would work the same way. Disk or covered wheels not so much because they look pretty cool regardless.

If you have to get down to actual theory, most sources claim a 2 or 3% reduction in drag with the wheels. I've tested a mcgyvered rear wheel cover and it was about 1.5% improvement of total drag so a commercial product would be that or better. Somewhat better with the front wheel. Your speed increase would be less than the drag decrease (the v cubed thing)