Not the sharpest thing in the world, but all else equal, way better than I could get with my Tokina 80-200mm f/2.8 and Tamron-F 1.4x teleconverter. And that combination generally required much higher ISO and slower shutter speeds.
The 70-200/2.8 with 1.4x converter should come out to f/4, which is 2 stops faster than the f/8 mirror lens, so in theory should require lower ISO and faster shutter speeds than the mirror lens.
Agree that it is not the sharpest sample I've seen from a cat lens, the good ones I've seen have been razor sharp. Not sure if that is due to atmospheric effects, the slow shutter speed (allowing movement of the moon and air current distortions), the lens, or the user (sorry, have to put this in to be safe) though.
Shooting the moon at its zenith will probably allow you to shoot through the thinnest cross section of atmosphere, you can find online calculators that will tell you what time that happens at your location. It may also be worth focusing in Live View to get the best focus - you might even consider hooking up your body to a laptop and outputting the Live View image to screen as a focusing aid if your body supports it. Use a cable release if you have one or set the timer if you don't. Also use mirror lockup as you are very sensitive to vibrations at 1280mm effective focal length. Shoot RAW, underexpose slightly and pull shadow detail/recover highlight detail to expand your (effective) DR. Lastly, I'd try to get the shutter speed as fast as you can to minimize effects of moon transit, airmass and tripod/ground vibration. Oh, and where you're shooting from also makes a difference. If you're shooting across an urban area, you can get stray light and you'll also have more air distortion due to rising heat waves.
Hope this helps!
Wind, Sand and Stars.