I'll toss my two cents in. On my Corvette, you disconnect the battery for a while and it will reset the learned fuel trims in the ECU. Known as long term fuel trims. Each ECU is different and the car/bike learns according to its environment, mods, and fuel used. I can see where disconnecting the battery would do something on the KTM. Obviously the dealer would have a diagnostic tool and wouldn't need to disconnect the battery. I can also see where idling it for 15 minutes may somehow put the ECU into a learn mode. Which would explain why things get better initially. Then as time goes on, the ECU is adjusting itself and things may get back to where they were before. Moral of the story is that without a proper tune for your mods, you may not be satisfied with the 15 minute relearn. I'm about to put Termignoni cans from a 998 on my 990. I'll do the 15 minute relearn and see how I like it. If it has undesirable characteristics I will tune the ECU. Right now I get 38-39mpg and would like to keep that the same or better.
2013 KTM 990 Baja w/ Termignoni cans
2013 KTM 690 Duke w/ performance cam, airbox, and Akrapovic full system