I think the levers have more to do with precision in the rear mech, anyway.
One one bike I have Ultegra levers and drivetrain but with an XTR RD (2001). It, of course, performs wonderfully.
On the other bike I recently replaced the rear lever (was a Sora, which wore out) with a 105 lever. This bike has an LX RD.
To be perfectly honest, to me there is very little (if any) noticeable difference in shifting performance between the Ultegra/XTR combo and the 105/LX combo in normal road riding on a touring-type bike. In critical applications, racing or precision technical off-road riding, maybe it would matter.
If I were building up a similar bike today I'd probably go with Ultegra and XT.
RichC
__________________
Training: 2002 Fuji Roubaix Pro (105 triple)
Commuting/Daytripping: 2001 Airborne Carpe Diem (Ultegra/XTR, touring wheels)
Commuting/Touring: 2000 Novara Randonee (Sora/Tiagra/LX, fenders, lights)