The secondary's dearth of talent, IMO, is something that can't really be gameplanned around, for the most part. At least anymore than it already is planned around. A stronger pass rush would make it less problematic, but that would require playing a lot of man-to-man coverage, and Arrington and McCourty are both a lot more effective playing zone.
So no, I don't think that Romeo would be a magic bullet, by any stretch. I'd take him back here, sure -- he's a very good DC -- but the defense won't become upper-tier until we significantly upgrade the talent in the secondary. At present, there's one guy back there who should be starting (McCourty) and then a few others who are either physically overmatched or inexperienced.