Dansk pre-muffler + SW chip: another data point
 

I know this is a tired old topic, but for those still interested, I installed a Dansk pre-muffler and a Steve W chip. The muffler is stock but has an extra outlet on the right side.
Installing the Dansk took two full days of hammering, cursing, heating, drilling and dremeling - as six of the nine bolts were rusted solid into the flanges. But the effort was worth it. The throttle response and acceleration were quite noticeably quicker. The noise level was only slightly louder and "harder," especially when accelerating. I have not noticed any of the burbling or popping noises that some on this BB have reported after installing a pre-muffler.
I waited a week to get used to the Dansk and then installed the Steve W chip. The additional power is not nearly so evident, except at around 4000rpm, when acceleration is markedly improved. The incremental effects must be a lot more subtle.
I do not regret buying the Steve W chip, but am surprised at how much more effect the premuffler had on the system.

Did you have a cat on before you installed the premuffler? It could have been in bad shape...