I think I'm about to do the same thing myself. I'm currently running a 11-28 105 5800 rear (running a 52-36 front).

I never use my 11 gear...I do not do downhill bombs and I'm too mortal to run a 52-11 any other time. Looking at the two, the lay out as such:

11-12-13-14-15-17-19-21-23-25-28

12-13-14-15-16-17-18-19-21-23-25

The 11-28 makes bigger jumps in the middle of the cassette...the place I spend the most time. The 12-25 adds a 16 and 18 tooth giving you a smooth, 1 tooth per shift set from 12 all the way to 19.

My only beef would be losing the 28, which I do use from time to time...Possibly I could cannibalize one and make my own...not sure though, the 105 larger gears are fused into on piece.