we discussed BBC percussion a few days ago, as oldmacilli said. Basically it is not correct to say that the Expression Map responds only to velocity as you can easily test for yourself by sliding up and down in the dynamics lane. However, it is true that the velocity response appears to be much more contrasty. The simple answer you need both and whatever you choose, you’ll probably need to set the dynamic manually for the majority of entries.
Looking at the timpani for instance, the default setting in the EM is CC1 primary +CC11 secondary full range. Without adjusting the velocity, this gives a somewhat restricted range. An obvious thing to try would be Velocity + CC11 but the trouble here is there is a sharp spike near the top of the velocity range so performance is more uneven and if you cancel the secondary altogether and choose only velocity, this becomes more extreme and is unusable in my view. The exact response will vary from instrument to instrument and indeed, as oldmacilli has correctly pointed out, from articulation to articulation. Don’t forget that in the instrument itself, there is a setting for dynamics which defaults to “full velocity range”. This seems to mean somewhat different things for different instruments in notation software but percussion certainly needs the velocity and I certainly wouldn’t recommend switching it off (i.e change to vel. mapped to dynamics).
I should stress that I am using the Core version of the library. I think it’s highly unlikely that the maps are different for the common instruments as the articulations are supposed to be the same but if we really do find discrepancies in the way things work, then we should compare the maps.