Advice on slugs for FX 700mm X Superior Liner (latest version)

What cal? 

I have had great results from H&N 25 and 27 grain slugs. I have had near identical accuracy from both .217 and .218s. These have been consistently the most accurate of the many slugs I have tested. The JSB KOs in .217 have been nearly as accurately. Currently, in my 700mm it is the most accurate slug at about 960fps but in a superior heavy liner. 
I have not tested Nielsen slugs though. 

I like the 27.5. Heavy enough and good bc. My mav is shooting them at 910

I’ve had good luck with JSB KNOCK OUT 217 diameters at about 935 tps out of my equipment ( identical to yours). DO not buy the 216 by mistake 

Rec’d my Crown MKll couple wks ago - set it up with 700mm heavy, Sumo, 9gm hammer weight, 175 bar and .925 hammer spring.

With NSA 31.2 .217, 5 went into .300” at 75yds at 1010fps. Repeated at .325”. Pretty good just out of the box - no ‘tuning’. 

It's worth trying out Varmint Knockers. I don't have a 22 cal FX but those Varmint Knockers are popping real good upon impact with my 22 Raptor (.218 22.5 grain). I just ordered the entire lineup - up to 40 grainers to test out. Will give me some tuning time on the Raptor as well, that way I can get more familiar with this rifle. I typically shoot NSA slugs out of my Raptor and 25 wildcat sniper.