I think that if you buff SCH to any meaningful degree, WHM loses their seat at the raid table once again. The question in progression raiding is ALWAYS going to be "what healer do we want to bring with our AST?" when Balance is as strong as it is and the job doesn't suffer significantly in other areas.

Remember, aside from Assize, every second a WHM is DPSing is one they're not healing. (Don't be pedantic, I'm aware regens exist, but AST has those too and they're fairly similar in strength.) AOE Balance doesn't have that limitation. If Omega Savage has high periods of frequent single-target or AOE damage then WHM's superior personal DPS drops off quite a bit in value because they have to spend GCDs to heal rather than DPS, whereas Balance only sees reduction re: the AST's DPS loss. Every other non-healer player will be continuing to benefit from that effect no matter WHAT the healer is busy doing in the raid.