"I also find it extremely difficult to downshift the rear cog...almost as if the shift lever is too tight...yet my LBS says it is perfectly fine and can see nothing wrong with it. Maybe I just lack the hand strength to move the lever with ease."

It's interesting you mentioned that b/c I wondered with that new design of the cables being routed along the handlebar if that would cause excess drag in the cables themselves? I have older Ultegra now on my bike and it is super fast and responsive. I guess I didn't immediately notice if the new Synapse was slower, should have paid more attention to it I guess. I really liked the clean look but if it makes it hard to shift then it's no good. Hopefully its just a break in issue with your bike? If you still have issues I'd keep going back cuz I know that bike wasn't cheap.