I always start with the (hand) impact driver if it is the first time I have removed an old jis screw. 10 bikes, and never ruined a screw that was good when I started. USE the impact driver first!
Now, to get that rounded, stripped screw out you may have to cut a new slot in it with a dremel tool and a skinny cut-off wheel. Then you can use the impact driver with a straight blade bit.
Editorial comment: if you are riding an old bike to save money, you have to invest in the basic correct tools and a FACTORY service manual. The money you save by not breaking stuff will more than pay for the tools. The money you save by not taking it to a shop will pay for the bike! Clymer gets their pics by copying the factory service manual so the pictures lose the resolution you need to see the details.
I have the bike you are working on, '82 cb650sc, it take 10 to 15 minutes to disassemble the seat and rear bodywork to get to all the wires and fasteners that you want to work with.