Sierra

Want to build a bike with 34/50 compact drive crank and 12-27 cassette. You guys told me that a front derailleur for a double should work fine with this. Now I'm wondering about the rear derailleur. I would like to use a Dura Ace 7700 for a double if I can.
The specs are as follows;

Largest cog 27t
Total capacity 29
Maximum front difference 14

I need a total capacity of 31, but I'm not too concerned with that as I plan to shift it as a crossover so will never be in the large-large, or small-small combinations.

What I'm wondering about is the Front difference spec(which I am exceeding). Why does the rear derailleur care what the difference is in front as long as total capacity is not exceeded?

Anyone else using this gearing combo? What do you use for a rear der.?

Thanks.