Not Alta but GTT, 17% which I'm using one step colder with, NGK BKR7EIX for example. Many people say not to bother getting the car remapped but I would say it's a good idea. Up to you of course.
No matter what others say, but perhaps consider the JCW models use different maps, and those of us fitting pulleys are generally putting even smaller ones on, and may be intending to drive the cars harder, so a good idea in my opinion although more money. Better to spend it now than on damage to the engine later perhaps?