No, that is secondary. Phase II is simply much more efficient. Period.
With Phase II comes TDMA, which in current generation is 2 Slot. That means 2 users active per TG per Voice Chan. P25 Phase I is FDMA, which only allows 1 user per TG per VC at a time. The sheer volume of usage on the current Memphis/Shelby County TRS is actually astounding, and the system really can't handle too much growth. An P25 can handle a LOT more RID's than the Smart Zone in use now. And then you toss in the fact that Motorola is no longer supporting the Smart Zone format. If the plan goes as it was originally intended for the Shelby County system, it will do much as Nashville in going to P25, then a certain other system will merge it. If Bartlett has a clue, they will also jump on board while the opportunity is actually an cheaper proposal for their options as well..
Keep your eyes and ears open folks, things are starting to get interesting again.