I already test flew the Alpina 4 in ML size, a bit further down in the posts.
Here are the S and MS sizes.
I flew both sizes, the S at 85 and the MS at 93 all up. The difference between the A4 and the Delta 4 MS for example is a better launching behavior, mainly due to the lighter materials. The A4 rises better than the Delta 4. The maneuverability and agility are the same, but probably the Alpina 4 has a slightly smoother feel, and the brake's authority seems lighter and felt a bit more linear.
The overall performance is the same as the Delta 4 in the same size, and also the speed. The S size feels naturally more dynamic, and quite agile. In turbulent air, the A4 seems to inform the pilot smoothly and accurately better the airmass than the Delta 4. Perhaps it is the light materials, but for me, I was really satisfied by the steering ability and feel of the A4. The differences are really small but noticeable...