Maco company in Germany sells a developer which works fine with that film named RPX-D. You can get real 1000 ASA from it, with decent shadow separation. I have tested it. You can see the results in
this pdf-file. "belichtet wie" means "exposured like", the vertical text "entwickelt wie" means "developed like".
If you use the minimum amount of developer (21 ml per film), which means developed like 400 ASA, you get real 800 ASA which is quite a good result. By doubling the developing costs (developed like 1600 ASA) you gain half a stop.
The grain is typical for a 400 ASA film and does not increase with forced development. In
this image you see a detail, the tree at the right edge, the upper part shows developed like 400, the bottom show developed like 1600.