I've got a Daystate Airwolf MCT with a Heliboard. Its favorite slugs are the NSA 20.2s in .216 diameter. I've tried 216, .2165, and .217 diameters as well as other slug manufactures. Larger diameter slugs just fit tighter and slow down more.
20.2 are 850fps
23.0 are 790
17.5 are 893 fps
23.0 and 17.5 shoot fine, but the 20.2s are one hole at 50 yards just like JSB 18.13 pellets.
My only negative on the NSA slugs is they don't expand at those speeds, but JSB 18s really don't either. I'll take the accuracy and bc though.
I've just begun testing slugs in my Impact MK3, so more on that later w/balistic gel testing. Maybe around 1050fps I'll see expansion with the NSAs I've got all the .22 NSA weights slugs, Javelins, FX slugs, JSB knockouts, and another manufacture I can't recall.