Here are some dyno charts of disc testing I did on a Supertrapp Supermeg exhaust I have on my dyna. It isn't quite a standard one though, but it still shows you what a difference a few discs make. I thought the megaphone was too long from the factory and it was too quiet for my tastes, so I made a shorter megaphone. There is about 4" of the perforated tube and packing material, just enough to take out the tinny sound. I am running the open end cap.

You can see with this engine there is as much as 3.5 hp difference at peak, and as mych as 6.5 ftlbs at 3300 RPMs. The second chart is a close up of where most people spend their time riding. You can see that the discs that make the most peak horsepower aren't necessarily the ones that are the best. I ended up running 13 discs and the open cap.

I found it interesting. Also attached a photo of how the exhaust turned out. I'm very happy with the way it turned out.

 

So you're running with 15 disks?

 

Just seat of the pants dyno I ended up with 16 disk with the open end cap. I plan to get it on a dyno in the next month or so.

 

What I understand from the Supertrapp site is more discs = high end performance, less discs= low end torque. That's consistent with your charts. They also say that an open end cap is equivalent to 10 discs. So 10 discs plus an open cap = 20 discs with a closed cap.
It might be interesting to try an open cap + 3 discs (the equivalent to 13 discs and closed cap) to see if you gain noticeable low end power.

 

Mine ran best all around with 13 open.. it varies with each build so you just need to play with them

 

I did some dyno testing too and found, simply, the more disks the better the performance. A low number of disks did not improve the performance anywhere; in fact the number of disks made little difference until 3000rpm. Optimum number was 26 disks. Open end cap knocked off performance no matter how many disks I used.

 

No, 13/open end cap. I did not want to give up that extra torque in the 3-4k range for the 1.5 hp at 5500 where you can't notice it anyway.

 

I know this is reviving a LONG dead thread but I hope the OP is still active on here.

I would love to chat you up about your supermeg set up. I just installed one on my 11 street bob.