An important thing to consider with the alphas is that -2.5 cm is the center of the running length of the ski (the part with normal camber). Since there is more rocker in the toes, anything in front of the recommended line will have a more drastic influence than usual. Its kind of like mounting a regular camber ski in front of center.
That being said its all personal preference and there's still the overall length of the ski to consider. I just picked up a pair and mounted them at - 1.5 cm. Haven't ridden them yet, but definitely stoked for snow to drop.