Just because a gun comes with a bad set of stock gears, doesn't mean you should pass up one of the most solid AEG bases ever made. Not to mention the best M249 on the market. They're upgrade platforms, you're replacing all the internals anyway so what does it matter if it has a hit-or-miss piston and weak gear axles? lol
Honestly if you're not pushing your gears very hard, they can run loose for a long time. It's more critical in helical gears since the nature of their meshing causes axial force.
But if shimming gears didn't improve performance, we wouldn't try so hard to get it perfect in industrial applications.