Certainly interesting findings. Really one would expect it to be fast while ARC is filling, then slow down as it is full - that is how traditional caching works. I guess we have a little rebel ARC going on at the moment. We are happier with master's ARC, but that doesn't mean we are entirely happy with it yet. Could be there is still some odd starvation going on during ARC filling.
Having said that, we are currently looking at huge changes done by rottegift, where we finally use OSX's UBC with arc, so everything is about to be different But with some nice sideffects.