I've heard about buying motorcycle tunes online by telling the dealer your mods and they can sell a PCV map but does anyone know a dealer that does this for the RC390?
You may want to consider either a different and wider clamp or a sealant such as Permatex.
https://www.permatex.com/products/gasketing/gasket-makers/permatex-high-temp-red-rtv-silicone-gasket/
I am wondering what are the symptoms you are observing that indicate an exhaust leak? I'm sure it may seem obvious, but I'm curious if it is visible exhaust, smell alone, check engine light or other dashboard indication?