I do see how if your sashes are in great shape and have beefy wood all around the critical corner area where the upper and lower sash meet, the chunk of insulation that's supposed to stop drafts passing through that vulnerable area should work pretty well. Unfortunately in my case, the corner edge of my lower sashes have that edge routed off and are partially worn. I'm assuming that was original, which is fine but that inner corner is where I still feel a slight draft get through. The fact that the original parting beads were also just a smidge deeper that the replacements means the new bead rests a little further into the channel than they originally did. So how to fix this shortcoming?
I plan to break out the epoxy and/or glue new strips of wood on these corners so they sit flush to the insulation on the new parting beads. I'm also considering pulling the beads back out and adding some 1/4" shims made out the old beads so they snug up nice and tight. I also wasn't quite happy with the weatherstrip that runs along the top rail of the lower sash. Maybe my sash aren't perfectly straight as the weatherstrip has a very short profile and you could still see a slight gap in some spots. To compensate I added an additional 1/8" rubber weatherstrip just above the easy stop strip. That works much better -- I don't plan to open and close these windows all the time so we shouldn't see too much wear and tear on the rubber stuff. Once I get around to fixing the present shortcomings, I'll report back if it made a meaningful difference..