I'm not a big fan of aftermarket exhausts. Every time I've tried one (for the looks and the sound, never for the performance, which is almost always lacking), I've noticed a loss of torque at very low RPMs, making city riding less enjoyable.
I did make an exception for my CRF1000, though, with a full exhaust system. However, I still alternate between the stock muffler and the Arrow.
Aside from a marginal gain at very high RPMs, the stock exhaust will always offer the best performance.
For the Pan European, I find the original silencers to be the best integrated into the lines of the motorcycle, and I prefer to enjoy the sound of the V4 rather than the noise of the exhaust.