Great Question!!!
The FullSac is not a fair assessment of just the SE pipes versus FullSac (or less restrictive pipe such as Rineharts, V&H etc). The Stock includes the restrictive air cleaner, cat header and the SE pipes.
My bike had a catless header, stage 1 air cleaner, PC V with auto tune and Rinehart slipon's. The noise after 40 years of riding was to much. Thus, I bought and installed a brand new set of SE 4" pipes to replace my Rineharts. As I supsected the SE pipes ran almost identical numbers on the Dyno. Horepower went from 93.8 to 93 and torque went from 111 to 113. The Rinehart numbers were when the bike was new and it now shas 21000 miles on it.
FullSac themselves admits the 1.75" cores and the louder 2.0" cores provide the best performance numbers over their 2.25" less restrictive cores. I have had D&D fat cats, Rinehart's and V&H on my rpior bikes. I think the 4" SE pipes are very good in not excellent performance wise. They just don't bark at you when get on the throttle and they are "wimpy" sounding at idle.
My high performenace 08 Vette gets better mileage when the muffler system in hte quiet more back pressure mode. Just doesn't sound as good.
Besides, how many of us ride the bike at the upper end of the RPM where the HP starts to matter. Torque is what we use most of.
I admit I am having to get use to a guiet bike because it's easy to leave the bike in a lower gear. My bike like running at about 2900 RPM for good torque and HP numbers. With the Rineharts at 2900 it was loud. Now with the SE 4" pipes, I can leave it in a lower gear and run at 2900 and not even notice it.
I road my bike "stock" with the exception of the Rinehart's. The biggest diufference was getting rid of the stock A/C and the damned cat.
