don't worry about toptube length, you should just focused on reach and stack because they are based on BB to stem, from my research, trek domane has the shortest reach with highest stack in a size 56 compared to Roubaix, look 765, defy etc. Even though domane has shorter HT than roubaix, but the extra BB drop makes up for it. it's always easier in increase the stack then shorten a reach. Smaller bike is also lighter and corners better due to shorter wheelbase. Lets face it, a 12mm stem looks much sexier than a 80mm stem