I do not know what size the baffles are in the SVT bone shakers, Big baffles can cause a lost of low end torque. Its best to have 1.75 to 2 inch baffles for the best low end torque, 2.25 do not suffer a loss in torque. However mufflers with 2.5 and larger baffles suffer a loss in torque especially in the lower RPM.
A good head pipe will help HP and torque if properly tuned. Fullsac makes a very good head pipe for the M8 motor, probably the best 2-1-2 head pipe available right now for a stock to mild build.
I can tell you, on my 2018 CVO RG with the fullsac MX head pipe, his 2.25 baffles and a good tune, I picked up close to 8 HP and Torque.
On the M8 the head pipe will be a little louder than just baffles but not a lot louder like a twin cam.
My 19 will get the fullsac MX head pipe when I can get enough put aside for a tuner.