Not very. Most of the studies I've read or seen referred to said that there is little aero benefit until you get up 38 mm of rim depth or so. Some of that research was undoubtedly funded by HED and Zipp, but I doubt all of it.

I think there's more to it than just rim depth. I seem to recall that John Cobb had tested Rolf Vector Pro's in the wind tunnel at one point and was surprised by how un-aero they were despite looking like a pretty aero wheelset.

Surprisingly, it's more than just rim width. Go on the Hed website and you'll notice that the Alps with a 50 mm rim are more aero than the 60 mm Jets.

There is/was also an old aero test comparing Campy Shamal (40mm) with Hed Cx (60mm). The Cx was slightly more aero in a face on wind but much more affected by cross winds. Conclusion was the Campy was the better "real world" wheel. I had both and felt the same way. Do a google and you may still be able to find the article.

Rim curvature plays a big role. Spoke count & spoke type is also a big part, which is certainly a big part of why the Alps are more aero than the Jet 60's. The 30mm rims are likely an effort to draw on the marketing edge of deep-V wheels. By all accounts, though, many of the 30mm rims are very strong (part of why they can use lower spokes), however, and that value shouldn't be overlooked if rigidity is important to you. But don't choose a 30mm over a 20mm rim because of aerodynamics. Aerodynamics should not be a concern for training wheels, and race wheels should be true deep-V wheels.

The lower-spoke count of many training wheels, however and the presence of aero-spokes on many training wheels (Mavic Kyriums -- 20 fr / 20 rr & bladed) will reduce the drag, however.

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