How much of a difference would you expect to see from a HQ ZnSe 101.6/4" lens make?
Over the standard 1.5" lens
Difference in what way?
A longer focus point because of a narrower angle of approach so maybe straighter side walls. Perhaps easier to get a narrower cut line.
Some loss of heat intensity in the beam per Ma output perhaps due to having to pass through more air??
Sorry I meant to say I’m cut capacity
With a longer focal length you get a longer depth of field which gives you a longer work length before the beam broadens out, like Steve said. You won’t be able to cut more in one pass, but you can do multiple passes to cut thicker pieces with straighter sides. Also the standard lens is 2" (50.8mm) not 1.5" 
On the stuff I’ve read it says the longer focal length can cutter thicker pieces, was curious if anyone had one for comparison
New machine has a 1.5"(40mm) focal length. According to the seller but I’ll have to see.