Anyone using a 2x TC on their M645?

I picked up a Mamiya 500mm Reflex lens last week for fun, and now I'm thinking of getting a 2x tc for it. Aside from the Mamiya TC, there are a number of other brands available. I know that (typically) a TC will degrade the image a bit, but I'm wondering if the Mamiya brand is better then the others? Any samples with vs without the TC?

I have at least one 2X in Mamiya 645 mount but have not used it much. Most 500mm mirror lenses are f/8 in speed. With a 2X you would have 1000mm to work with but f/16 to focus through. Even with a plain matte or grid type focusing screen that would be difficult. My longest Mamiya 645 lens is the 300/5.6. A 2X would make that about 600mm f/11. Each teleconverter will work differently with each lens. These don't cost very much now so you can experiment.

I have a Kenko Teleplus MC6 for mine which I have used with my Zeiss Sonnar Pentacon 6 lenses. It was superb with the 300 f/4 which is where I most used it. I haven't used it that much with the 180 f/2.8 because I have the 300 f/4. I would expect that starting with an f/8 lens focusing at f/16 could be difficult.

Thanks.

Fortunately, I'm in Florida, and even at f/16 there is usually enough light for me to be able to focus. Is it the easiest? No. But I have been successful most of the time.

Seeing how they're so cheap, I may give one a try.