@UCDHacker, I just went through comparing the "race" road bike geometries in Cervelo, Canondale, Trek and Specialized
I didn't exactly write down detailed conclusions, but the notes I wrote were that Cervelo and Trek had the highest aspect ratio (relatively tall compared to length). Canondale was a little lower, and specialized was the lowest. Problem is though, I was looking at the larger sizes. Things sometimes get funny in the smaller sizes as they all compromise differently. Just check out the geometry tables and see which ones have lower stack for the same reach in your range.

However, you might have to look at the photos or try the real thing too, because stack doesn't necessarily tell you the standover height, if the top tubes slope differently. My guess is they all slope pretty similarly though.

Hitting the top tube seems pretty wrong to me on a modern style bike, and may just be a size (or two) too big. One thing I've been really realizing, maybe kind of obvious, is you can't tolerate nearly as much saddle to handlebar drop in a fit that's too long. For the same body position, your arms swing in an arc where up means out. Down without coming in will put you in a lower body position, for arms extended on the hoods at least. The problem on those smaller frame sizes though is they don't tend to get shorter in length as they get lower. If you're really right about your problem though, that could be good for you. A size smaller is likely the same length, but lower.

Realize though that length is determined by REACH, not top tube length. The smaller sizes usually do have shorter top tube length, but the reach will often be the same. For the smaller size you need more setback on the saddle post to get the seat in the same position relative to the BB, but once you do that, as you should, the length of the bike is the same. You may just need to adjust the saddle position on the 50. You can of course also add a couple of cm to the stem.