relative to gearing, and I know someone will tell me I'm way off (with the facts), but my "seat of the pants" experience was that at 100-120 I can cruise several miles and I am probably not cracking 5 grand, maybe 5500(?) I could be wrong, but the bike doesn't even seem stressed at those speeds/rpm's.
also, 80mph is an "all day" cruise on the xb12's (IMHO).
as for PA 43 - it's a cool road from an engineering perspective, it's got these darn toll booths in the middle of it. it is desolate on a sunday and high speeds can be attained. however, there is some PoPo presence on there. i would not advocate making it a place for high speed runs. my little experiment was a once-only type of thing.
@upthemaiden: best place to get 43 is to ride down to Glenwood Br via 837 up past hot metal harley and take 837 and make a right on "_______ Hill Road" at the bottom of that hill you connect directly to the entrance to pa 43 and you can ride it about 15 miles or so of flat, smooth, wide open road with huge runoff areas and 100 yard sightlines on either side of the road. perfect for, well, you know....