My Ultegra brifter requires a lot of effort to shift down from the large chainring and clanks when it finally shifts. This came on suddenly, during a ride, prior to which it had been shifting smoothly, with light effort. I tried loosening the upper limit screw, thinking that perhaps there was too much tension on the cable when trying to downshift, but that seems to have little to no effect. Next step is to take it to LBS for an expert to fix. Any suggestions before I do that?

Lube your cable and brifter internals. Could be a bit of gunk clogging up the release mechanism in the shifter itself.

Edit: also lube all the joints on your front mech after spraying some WD-40 in there whilst moving the cage by hand.

If the cables are around 1 year old, replace them because if it breaks in the shifter, then you have a bigger problem.

Sounds to me like you need a new cable and housing. Disconnect the cable from the derailleur and than **** the brifter. If the shifter is hard to move replace the cable and housing. If the shifter moves easily your problem is the derailleur.

Look for possible kinks in the cable and housing.
Check the cable guide under the bottom bracket. Clean if needed.
Shoot WD40 into the shifters.
Moving the limit screw was not the thing to do. Adjust the screw according to instructions found at www.parktool.com.

Are you running triple or double chainrings?

Sounds like the FD got twisted on the tube (it's a clamp on, right?), bent, or the cable got pulled out a bit. Loosen everything, reallign the FD cage, and adjust and tighten everything the way you would when tuning up the shifters. Also check the chain and chainrings for wear. Why did the chain come off in the first place?

Lubricating all the pivots on the FD improved things immensely. Thanks for the tips.