Pressure and lowering performance, in itself, will not damage the engine, the FUEL charge, being too LEAN; will. The leaner the fuel mix, the hotter the engine (combustion chamber) runs, PLEASE look it up before you say it is not true, because it is. You can't force more air than at atmospheric pressure without a super/turbo charger, so that will NEVER change. Air flow and Air Pressure are different.
Back pressure is a generic equivalent to valve overlap, which gives you more low end power, while lower back pressure does just the opposite, and changes the torque curve to higher RPMs. Changing the size of the pipes, larger, lowers back pressure, or opening up the mufflers does the same. Good mufflers and smaller pipes create back pressure. Mitsubishi and Dodge use back pressure to change the torque curve on their cars - EXAMPLE: I have a 2000 dodge Avenger ES, and have had a 2000 Chrysler Cirrus LXi. BOTH have the exact same Drive Train - 2.5 V6, and A604 trans, but the CIRRUS had a little more get-up and go power, while the Avenger has longevity. I looked at the Exhausts, and there was an extra piece on the exhaust my Avenger does not have, like another muffler or resonator. THAT little piece creates a little more back pressure on the Cirrus, thus changing the torque curve to about 100rpm lower than what my Avenger is. 4-cycle engines all operate on the same idealism.
The only way you are going to damage your MC engine is if the fuel charge is too lean, because a V-Twin works twice as hard as an auto motor, since there are no 2-cylinder cars people DRIVE. The heat they generate is enough as it is, let alone a lean fuel mixture. The FI engines self adjust to a certain point, but you would need a external processor to help remap air and fuel, if a lean condition occurs due to too much air flow. These computers are for sale on Ebay, at about 200.00 though.
Harley motors are made to run lean, so they do not have to have PAIR systems like ours do. They run hotter than foreign bikes do, even though most foreign bikes are water cooled now. Harley is too stubborn, and won't water cool their engines... THREE people I know who have Harleys won't run them in 80 degree or above weather for fear of overheating... This has been a problem that has plagued Harley for years, so the Japanese redesigned the motor for them...
ANYWAY, you would have to radically change your system to like NO back pressure to cause damage to the motor. NO back pressure at all, would be like having bad compression rings, you are not going anywhere fast. I just bought another Exhaust system from a 2006, for 20 bucks cash (minus the front header pipe that my uncle bought two weeks prior) I am going to remove the rear exhaust plates on mine, and leave the baffles in for now. IF I don't like it, I will put the new mufflers on, and sell the modified ones. I lost MPG on my Vulcan doing this, so we'll see what it sounds like, and performs like on the FI system -vs- a carburettor system.