Looks like about 1.5hp considering the slip-on only added 1 hp according to this. I weighed the catalytic converter and it was slightly over 10 pounds. So 40% will be a 4 pound weight savings.
Arrow’s exhausts for KTM SuperDuke 1290 have just been released: for the “super” SuperDuke X-Kone and Race-Tech silencers are ready, street legal with removable baffle, to be fitted on KTM OE pipes or with Arrow’s “de-kat” mid-pipe (this can be also used in combination with KTM OE silencer). These systems give interesting performance: fitting only the silencer we measured 155hp on the wheel, with the “half” kit we measured 156.5hp (OE system stopped to 154hp), torque is really better at low and mid-range. Weight reduction is about 40%, depending on the versions installed.