Jim,
When I did the Fullsac conversion, I already had true dual headers, A/C, etc. The bike had been dyno tuned with different mufflers, but both were fairly open, free flowing styles, so I didn't notice any difference in tuning. I eventually took the fullsac's off though as they were a little too loud. Looked great, sounded great, but Steve had led me to believe that they would be quieter than most aftermarket mufflers, so they were just louder that what I was looking for. Currently I'm still using the true dual headers ( V&H ) but with supertrapp tuneable megaphone disc mufflers and like them very much. FWIW, I have 18 disc's on each side. Started with 12, but that was too muffled sounding.