At the same pitch. 27" baritone is 24.75" plus two more frets. 28.625 is 25.5"+2.
Each 0.001 is roughly equal to a half step in tension.
A 28.625 sporting 8's at standard pitch (E) is roughly the same tension as 0.010's tuned to STD on a 25.5. I put 14's on my acoustic, and tune to C#. So it plays like 13's on a regular scale at concert pitch.
So you should be able to put 7's at Eb or D and still have something close to the tension of a 9 at standard tuning on a standard scale.
Which is probably where you would want 7's. They have a narrow tuning range, and trying to sing to guitars in E is why the castrati once ruled the airwaves as lead singers.