Flirting with "house ruling" that units of 10+ get a 12" coherency range rather than 6" ( either that or use the optional speed play squad leader and radiate the coherency from the leader.) After playing some games I think we'll be using the squad leader designation and having all models in squad obey the 2" coherency rules and all models in unit must be within 6" of squad leader ( effectively giving a squad a 12" deployment diameter.)
i found that 9" fairly easily accomodate a 20 model unit, maybe if you have 40mm bases on such a huge unit it could be problematic, but i cant think of a unit for which such large bases would be reasonable that can be doubled up above 10
One additional tweak we've made to the coherency rule is to use squad leader rules. Since the unit leader ( or attached hero) is the ultimate in bubble wrapped in GF, for the sake of counting objectives, only the unit leader's position relative to the marker is measured. This prevents large units from trying to poach multiple objectives at the same time
I managed to play my Orc Marauders with 20 Boys + Warlord units and it fit exactly into the 6" bubble. Though I have to say it was mainly possible because I have my 40k Orcs still on their old 25mm bases. If you have a new orc army based on 32mm bases (as is now GW standard) it does not fit into the 6" bubble. The 9" bubble works pretty well though with the new bases. If you allow for a 12" bubble it is possible to hold to objectives with one unit. From what I gathered the idea with the 6" bubble is to avoid this from happening. For our gaming group we adapted the 9" bubble with the additional rule to keep them as much bunched up as possible. Kind of a gentlemans aggreement. Works pretty well for our group.