Plasma 5.9.2 on openSUSE Tumbleweed. The Multimedia > Audio Volume systemsettings module lets me manually raise the system volume all the way to 150%, and this works fine. However, when I use the volume keys on my keyboard, it tops out at 100%. TO go beyond 100%, I need to open that settings page and adjust the slider, which is a pain in the butt. I'd really like a checkbox or option in that settings page to raise the top volume level when using the hardware keys to whatever the actual system maximum is--150%, in my case.
Never mind, this is already possible, albeit in a slightly roundabout manner: right-click on volume plasmoid, set max volume, restart Plasma.