I admit up front that I may be over-thinking this, so feel free to disabuse me of this notion.
My two main bikes are a 2011 Specialized Allez that I built up from a frameset, and a 2010 Look 585 Optimum that I also built up from a frameset. Both frames are 61cm. The Allez gets the majority of the miles, since it's got the PT wheelset and fenders.
When I bought these two framesets, I was coming off my 58cm CAAD9. I had just had an expensive fit session, and ended up with one of those funky cheap adjustable stems to get my bars high enough. It offended my senses enough that I bought the 61cm Allez frameset (230mm head tube) and transferred the parts to it. That HT is high enough that I had to flip the stem to get the right bar height. The top tube is long enough that I had to go to a 90mm stem to get the right reach. The Look's dimensions are a bit smaller; in fact, it's smaller than a 58cm Roubaix...go figure. I've got no plans or desires to replace the Look. I've gotten both more fit and more limber since I got the Allez. I've since lowered the bars a bit, and am still comfortable in the drops for as long as I want to stay there.
The 2013 58cm Allez frameset has a 205mm head tube. Based on my figuring, I could be in the same position with a 10 degree stem flipped up and just the cone spacer on that frame. I'd also be using a 110mm stem. There would be a small weight advantage to the 2013 frame as well, since the 2013 fork is all carbon, where the 2011 fork has an aluminum steerer. A disadvantage may be that I might need a new crankset; I don't know whether the Specialized "over-size bottom bracket (OSBB)" would accept my standard crankset.
Would there be an appreciable advantage to going to the smaller frameset?