Dura Ace 7800 casette
 

Riding Dura Ace 7800 group. My cassette needs to be replaced and the Shimano spec calls out that the 10 speed 11-28 requires a 7900 derailleur. running a 50 34 compact crank and current cassette is a 12-27. Does anyone know if the 11-28 will work? Not sure if the issue is on either side or both. And maybe that extra tooth in the rear (on the low end) doesn't really make a difference?

It's likely an issue of chain wrap capacity.
(Largest ring-smallest ring) + (largest cog- smallest cog) = wrap capacity.
The 11-28 has a 17T difference while the 12-27 has a 15T difference. So the 11-28 needs 2T more capacity.
The RDER may not have enough capacity to "snug up" that last little bit.

It's really not a problem unless you ride small:smallest and even then, you just have a saggy chain.

I remember putting a 11-28 cassette with a 7800 rear derailleur; it works, but it's tight and makes a little noise when running the 28 because the upper pulley is so close to the cog. This is stock form -- no longer adjustment screw, or shimming the derailleur outbound or other common tricks to make this kind of thing work better. I didn't have a problem with chain length or slack.