The efficiency of this size is 66%, see column “H” at this webpage: http://www.roton.com/Engineering_Data.aspx?line=Hi-Lead
The efficiency is the same for steel and stainless steel. Efficiency is a function of lead angle, flank angle and coefficient of friction only. Here’s a link to the formula webpage (see # 10): http://www.roton.com/formulas.aspx
The preloaded nut is de-rated because the preload is very light and only effective for small loads. There is also a limit due to the flange mounting. We do have users who load these beyond the catalog rating of 25 lbs. without any problems but each application has be to tested to make sure it meets the service life and performance requirements.
You can use two sleeve nuts and make a more robust anti-backlash mechanism if PN 19664 proves to be inadequate.