No the widevine keys are totally unrelated. the problem is there is no hardware-level decoding of Amazon/Netflix streams with Kodi... at least not for Amlogic devices anyway, I can't speak for other platforms. You may be able to bump it up to 720p but I don't know as I don't have an Amazon account, I'm sure somebody here with one will be able to answer that question more than I can.
Ahh gotcha, thanks.