I'm going to go for the Empire exaust, you sell one?Maximim power isn't with either of those. Mixing brands is not a very good idea either. The HMF has a pretty decent midrange, but isn't great higher in the rpms. FMF or Pro Circuit made great 2 stroke pipes, but their 4 stroke stuff is middle of the road at best. The most powerful Z400's I've had on the dyno, or seen dyno sheets for, have had an Empire Industries, or a Barker's Performance full system. I'm a dealer of both, if you need any help finding one. That's not to say that there aren't some others that are close. Sparks is right up there. The old Alba system was decent. Yoshimura was pretty good for being one of the quieter pipes. Rossier has re-designed his stuff recently. I haven't had one on the dyno yet.