I am not a fan of loud bikes at all, especially those that rev it up as they go by your open car window, not mentioning any particular riders of course ( cough.....hardly davids......cough).
My Hayabusa had M4's when i bought it, they lasted about 6 seconds before i was installing the stock cans. No need for weight savings on that bird at all.
I've replaced the stock silencers on both KLR's for several reasons, namely; lighter weight (5 - 7 lbs)
That is my thought as well, just the weight savings on the KLR. I bought the Q4 and power bomb for it, read that it was the quietest of the aftermarket pipes as long as you keep the packing in good shape.
Well . . . I don't believe aftermarket mufflers add significant power increase across the rpm spectrum in comparison with OEM mufflers; certainly, with nominally stock engines.
As to, weight-saving, a few pounds to a 400+ pound motorcycle won't produce significant handling improvements, nor mass reduction, seems to me (a half-gallon less fuel in the tank will do the same weight reduction).
I honestly dont think anyone really believes they add more power, they just sound like it. As you mentioned they may rob power. Hoping that with the KLX jet kit i can just keep close to stock power.
Yes 6 or 7 lbs does not sound like much but it is almost a gallon of fuel and when one is on a long road trip loaded with a full complement of gear any weight saved is a plus in my book. Certainly going to carry all the fuel my 6.6 IMS tank can carry and more than likely too much gear. Lol
Just my thoughts