Martin having given them an end-point outline will undoubtedly have left miles of wiggle-room. All he has to have done is say "this person ends up on the throne" and they can take wildly different routes to get there.
GRRM and the writers spent about a week going over the future of the series last year, so I expect their outline of events is a bit more complex than that, but at this rate the last two seasons (particularly the seventh, which will have pretty much no written material to go off of; with the sixth I expect the writers will get access to whatever Martin has finished by the time they start writing it next year) will have broadly the same outline and the same final outcomes, but the texture will be different.
Allyn Gibson wrote:
For instance, Martin could bring on a cowriter to do some of the heavy lifting on Dream of Spring while he wraps up Winds of Winter alone.
Given GRRM's writing style, I don't think that's an option.