You're going to get different opinions, I can only base mine on my memory of the Steady Aim testing we did for XIM APEX. You'll find some posts from me about Steady Aim testing with XIM 4 and XIM APEX in Black Ops 4 with more info.
Do whatever you want in regards to DPI, Polling and Sens, but lower DPI of 1600 or 3200 would be my recommendation. Others will suggest 12K+ DPI. Try either Response Rate 125Hz, or Sync Default.
Depending on your DPI, you might hit the XIM APEX Sensitivity cap of 500 if going for RR 125Hz, so Sync Default will provide more flexibility and feel about the same. Try RR 125Hz, or Sync Default (or both) and go with whatever feels best.
Try Steady Aim of roughly 3 and tweak up or down a little. This is the key to getting that SA3 feel. Or search for OBsIV's Stairstepping curves. The curves most accurately emulate XIM 4 SA FW behaviour, but Steady Aim on APEX is more customisable.
No Boost or Smoothing, but that's up to taste. Smoothing's at the opposite end of the spectrum to Steady Aim (quantisation) and it's probably counterproductive.
