slow Ultimate SD upscale in SD.Next, tile artifacts abound; can't find MultiDiffusion among extensions

[minsky91]

I use Ultimate upscale and SD upscale extensively, as well as MultiDiffusion, both in Automatic and Forge. In the description of Ultimate upscale in SD.Next I read that it has 'more advanced options' and 'less artifacts than original using higher denoise ratio 0.3-0.5'. I have just ran a Ultimate SD upscale for a 7680x4320 image with 0.4 denoise and Scale 1 (just a refining pass), and the result was heavy with visible tile artifacts, much worse than I had it under Forge or Automatic. It also took considerably longer: 21 min. against 10 min. in Automatic and 4 minutes under Forge, with the same file and default parameters. Maybe I am missing some optimization parameters?
... Could SD upscale be a better choice under SD.Next, than the Ultimate one?

Another related Q: I have MultiDiffusion extension installed in SD.Next, but it doesn't show up in extensions. Where do I find it??