to the OP:
Your Specialized Roubaix Comp is the one equipped with all Shimano 105 components, correct?
If you want a lower gear ratio, the most common modification I've seen with 105 and Ultegra setups is a 32T cassette. While that will not give you a gear ratio quite as low as your Trek's 28 / 32 combo (0.875 gear ratio), going down to 34 / 32 on your 105 bike will give you a 1.06 gear ratio, which is a significant drop from your current 34 / 27 combo, which has a gear ratio of 1.25.
34 / 32 will give you a gear ratio halfway between your Trek and your Roubaix's current setup, and I have a feeling that would be "good enough" since your Roubaix probably rolls more freely than the Trek.
The Shimano 105 medium-cage rear derailleur (5700GS) will work with a 32T cassette, and it's an easy modification to make. Just make sure the B-tension screw is properly set so the RD pulley will clear the biggest cog on the cassette, and you will need a longer chain to accommodate crosschaining the biggest chainring and the biggest cassette cog.
That's what I'm running and I have no problems. 105 shifters, 5700GS rear derailleur, and an SRAM PG1070 11-32T cassette.
Hope this helps.