I just replaced my White Bros E-series silencer with a Pro-Meg headpipe and silencer. I had been happy with the slip-on/stock header combo until I added a 10.6:1 86mm JE piston, HRC camshaft, and FCR carb. It just didn't feel like there was enough flow through the exhaust to get the most out of piston, cam and carb upgrades. I tried all sorts of jetting and none of them would let the motor rev out very well. The Pro-Meg has changed that situation noticably - it revs out so much better now. The sound is a lot different too. Instead of the deep thumpy sound of the slip-on I have a higher pitched sharper sound- strangely it sounds quieter, but that could be due to newer silencer packing or some hearing loss on my part.

 

The Pro-Meg is meant for built motors and the slip-on for stock motors. I am also thinking about getting the WB pro-meg for my EX.
Thanks for the information.