Why don't you try swapping the seat from the Mercier to the C'dale? An uncomfortable seat is not going to help you faster. Neither will a "twitchy and stiff riding" bike. Don't discount the influence of discomfort on performance.

You say the gearing is close, but it's not really if you compare a compact double to a triple. Do you use the granny up front? If you do, not having the lower gearing with the double would account for your greater perceived hill-climbing effort. Bigger difference than 27 vs 28 at the rear.