Am I right in thinking that our gs 500 carbs only have a fuel screw or air screw as other bikes I have owned have had both the fuel, air mixture screw. I just need this confirming for when I do the lunchbox mod or is it not worth doing the mod with an upjet ?

You'll need to rejet. You have to drill out brass plugs to access the mixture screws. And you can control fuel with washers/orings on the main needle. Tons of threads here about it.

I say do it!!!!

The bikes run lean coming from the factory.  Those brass plugs are on the bottom of the carb, sealed up to keep our grubby tinkering fingers away from the screws.

My bike ran like a slug when I first got it.  I did the lunchbox, new exhaust, upsized a couple of the jets.  I also lopped off a bunch of other parts off the bike, so not sure it's all attributable to the carb mods but the new bike is such a blast to ride!