Trek 7.1 FX has one glaring weakness, the rear derailer package its?

strong suit, does it make sense to dump the standard transmission package initially and add the Shimano Ultegra? What I am reading and seeing is that Trek puts places $11-30 Shimano rear derailers on the lower tiered price points. This is proving to be problematic to the new customer who really knows nothing about how all this stuff works. I would expect a fast, smooth bike in the 7.1 FX. I want the ability to go 4-37 mph and when I shift it doesn't sound like a train on a track carrying a load. I want snick, snick whoosh! I am fairly sure this is what I want, but not sure if I should do it. To every action their is a reaction, I don't want to make a change, then be faced with rebuilding the bike either. Please advise. A 4 to1 gear ratio pedaling at 120 rpm will get me up to 37 mph. This is the type of capabilities that I want with the 7.1. What do you think?