For a faster but not elite level guy, is there a noticeable advantage to one wheel over the others?
tl;dr version: Yes, but it doesn't matter.
The long version is Maybe – it depends on the advantage you’re looking for, and it depends on your build, power, and your track (steepness? Indoor/outdoor?). Some people – especially big, strong people who ride on steep tracks – notice a benefit from the stiffness of wheels like the iO.
Aerodynamically speaking, you probably won’t notice an advantage. The biggest aerodynamic advantage is moving from standard low-profile wheels to a deep, U-shaped rim like the 808 or a 404. That’s the biggest jump. Moving from an 808 to anything else probably has a real but negligible effect. You can measure this using stuff like GoldenCheetah’s “AeroLab” if you have a power meter.
It never hurts to look at what pros are using. Sure, at World Cups it’s iO/Comete as far as the eye can see, but at lots of other UCI races, and at six-days, you see plenty of 808s, FFWD F6Ts, and similar wheels.
Bottom line – if you want to buy an iO, go for it. But if you want to feel as though you have great gear already, I hereby give you permission. Cause you do.
I say that as somebody who has perfectly good wheels (FFWD F6T) but just picked up a Corima 4spoke/disc combo.