Ken,
2.5" should be fine . My zone is about the same as you. I have been mulching since early Sept. but will have to start picking them up because they get far too deep to leave on the grass. I'll use my Troybilt vac to pick them up and compost them for future use.
I agree with you on backing over the leaves. I've found that whether mulching or bagging, the mower does a better job going backwards. My mower(LB10323) has the factory blade which seems to do a good job. I like to give the lawn a low mow at about 2" after all the leaves are down. It makes the lawn look neat for the winter.

Bogman, glad to see someone has noticed the same results (going backwards) with the LB. I am looking for a used SP mower just for leaves. After a bunch of googling seams like a Toro or Snapper Hi-Vac w/ gator blades are good at mulching leaves. LB works good going forwards & backward mowing from right to left but the pull backwards has left me sore at times.

I think the mower is mulching more than it can handle. Try slowing down or cutting a somewhat narrower swath. Going lower will add more cut grass to the mulch material.