Personally, I think you could even go some further with the bonus, but make it extremely rare ... like, it's not for granted to find a legendary even in 100 in game hours, but if you find it, it has a bonus of maybe 50% oder 75% over a regular weapon.
I don't see it as overpowered, after all the legendary weapon alone doesn't give you a legendary armor, so you are still easily vulnerable.
That said, you've gone to some extreme with the demolisher (a bit too far in my opinion, if having no base and fight on the street is much more resource efficient than having a base) - with a legendary, 11-20% seems too cautious in my eyes. It would probably just feel like a tear 7 weapon instead of something really special. In addition to the stats, a specific paint or minor design modification might make it stand out from an optical POV as well.
Just my two cents.