I believe Z51 will probably be constrained throughout the 2015 MY though the percentages will vary from the low of 55% now to some higher number as the year goes on. Basically I think the number will go up and down throughout the year as orders start to stabilize. The reason I believe it will remain constrained is GM has commitments from suppliers at the 65% level (according to comments from members of the Corvette team) over the course of the MY and will adjust constraint as they balance the orders.
This shouldn't mean getting a Z51 will be difficult. Last year it became a problem because they completely constrained the option early in the year which set of a buying frenzy even when they eased the constraint later on that carried on into the 2015 MY.
My prediction: By December you will be able to get whatever you want, probably from stock at a discount aside from artificially constrained markets like CA, NV and FL.