dirt bike dave, thanks for the answer.
i'm from Russia, and we have no problems with approval/legalization for now :D
you said that it would influence the rpm range. please explain, how?
i made a weight comparison today. 2.2 kg for the stock, and 1kg for the ktm12sx arrestor. Not the great difference, but the sound may become annoying on long trips for the second one. Moreover, the diameters of pipes are not compatible. so there's need to use some imagination to fit it. So probably it doesn't worth it.