the difference is in gearing. Given the same cassette, the 54/42 will be harder to push but you will go faster at a given cadence and gearing. your average speed should be about the same however. climbing will be slightly more difficult with the 54/42 (the small number is 3 teeth bigger) as far as the ultegra being better, i would say not much better. no you will not wreck anything. those cranks are rated to a 400 lb guy with a higher power output than anyone at the TdF.