Join Date: Apr 2015
Location: Murrysville, PA, USA. A pleasant, wooded suburb of Pittsburgh.
The creators of the 390 Duke, KTM, designed a compact, mass-centralized exhaust system that was carefully synchronized with the ECU's fuel mapping. As MalJohann notes, any change to the exhaust will perturb the air/fuel ratio programmed into the ECU: and this could lead to an excessively lean condition which could seriously damage your motor. KTM has approved the Akropovic exhaust as an aftermarket upgrade. But to use this exhaust also requires the ECU remapping reflash KTM offers.
I think your best bet is to leave the well-designed stock exhaust alone and not worry that it has a civilized exhaust note, as Mechanical_Vandal advises. A 390 Duke is never going to rumble like a Harley!
The Duke exhaust is one of my favorite features of the bike. It is centralized, compact, unobtrusive and effective. It does not require hanging a conventional-style can off the side of the bike - in addition to requiring a remap, this arrangement interferes with saddlebags, precludes (or burns) a pillion, creates extra aerodynamic drag to slow you, and is waiting to be mangled if there is a crash. The "cheap/safe/good" exhaust you are seeking is the one you already have!
"If you don't know where you're going, you might wind up somewhere else."
2015 390 Duke
1980 Moto Guzzi V50 II (becoming a cafe racer)
1984 Moto Guzzi SP 1000