These are all good points, and I'm going to agree with you to give me added motivation to drop weight from around my middle. If I can loose 20lbs I think it will be ok to put me (at 170lbs) and 20lbs of gear on a Roubaix. The one change I'll probably still make is an Al seat post as the 13 lbs of the saddle bag will have a bit of a moment on that post rather than weight from the rider that pushes straight down.
Nun, if you get down to 170 then you and your gear will be 15lbs lighter than I am on my own, and I wouldn't have any worries about the Roubaix wheels taking my weight, so imo you don't need to change them. As for gearing, a 50/12 is plenty enough to exploit the potential of the Roubaix as a road bike and a 34/28 will get the new slimline you up most hills, so to that extent I agree that you don't need to modify this bike too much.