I'm not at all convinced that a "light" tourer would make any difference. You're talking about 5 lbs difference in bike weights, more or less. Assuming the motor is in decent shape, it's not going to amount to very much on the road, unless you are trying to set personal bests.

Recently, I've started riding my LHT on my Saturday group rides. I have few problems keeping up with the roadies on 50 - 60 mile rides. I did ride a semi-organized century a few weeks ago, and noticed that I was NOT able to match my "Litespeed" pace, but the difference was only slightly more than a half hour. The lower gearing on the LHT compensates well for the extra weight.

Interestingly, my neck and butt hurt more after the century on the LHT (conventional touring geometry, Brooks saddle) than they do when I ride my Litespeed (aggressive geometry w/ San Marco saddle 3 1/2" higher than the bars) for similar distances, but that probably is attributable that my body is more adjusted to the Litespeed because I ride it more frequently. I am going to shelve it for the rest of the winter and stick with the LHT and see if it makes a difference by Spring.