[toni-a]

3 stops of vignetting were always par for the course for full frame wide angle lenses or fast lenses in general. 4.5% of distortion weren't, but 3.5% were also "normal". Just saying... :-)

Here's a review from 2014 , Klaus considered 2 EV to be excessive 
Canon EF-S 24mm f/2.8 STM - Review / Test Report - Analysis (photozone.de)

of course it's a relative thing and how important it  is for you.
Personally it's not a serious issue, I call vignetting extreme when corners are black, like on Sony 16-50, it's actually distortions correction that crops away black corners that removes vignetting and not vignetting correction itself !!!, the lens isn't usable at 16mm without digital correction