31mpg? Yes, if only by confirmation bias.
Assuming you stick to paved roads, the lower limit is set by speed bumps and the curb cuts to where they sell cheap gas. It triggers cRIPPLErOOSTER{sp} when people slow down for speed bumps. ...so, watch out.
There are certainly other benefits. If you only lower the front you have a better view.
Further to Frank Lee and twj347's comments: I have Macpherson struts but rear wheel drive on the Superbeetle (so YMMV), but I tried adjustable struts with stock spring rate and stiff as a Cockford Ollie. I combined lowered suspension with 50-series tires in the stock width. And a stress bar:
With the stiff springs if I drove with my elbow on the arm rest in 15 miles my collarbone would start to complain. Reverting to stock rate again ($$$) and the only downside is excessive body roll on those two lane left-turn freeway onramps and ground strikes on the edge of bridges on the Interstate.
To quote the skateboarders, "Let it grind".