Well, that's not exactly correct. Diesel automotive and truck engines do tend to have more power at any given RPM than gas car engines, but the diesel engines they're using in these applications tend to be little 4-cylinders that rev significantly higher than their Continental and Lycoming counterparts. What the propeller speed is varies depending on the gearing in the gearbox. Diesel horsepower is the same as gas horsepower, it's the other specifics of the engine. The SMA diesel is the only commercially available one I'm aware of that is actually direct drive and revs pretty low, but that's 2200 RPM, which is pretty close to in line with what a lot of the big Lycomings turn. 2200 RPM is a standard cruise point on my Aztec (although I'll run higher to get more power depending on the altitude I'm flying at).
Also, these little diesel engines run insanely high boost, and the turbos aren't able to maintain power at altitude the same way as gas engines are, because the turbochargers simply can't get the sorts of pressure ratios up high that you'd need to maintain that. So, you aren't able to keep that power all the way up to altitude, unless you get into some sort of sequential turbo setup. So, these turbodiesels tend to be more along the lines of somewhere in between a naturally aspirated and a turbocharged gasoline engine, but their altitude performance is still not as good as a turbocharged gasoline engine. It might be able to keep up in cruise under certain conditions, but that would be running the thing firewalled, and assume a low power setting on the gas engine. Your maximum performance figures would suffer quite a bit.
All of these comments ignore the hurdles of getting the FAA to give their OK on it, which takes a lot of work.
