yeah, as Tony says, purely for max power setting, I'd consider JSB RS / express / exact but also try with superdome as the larger, tighter fitting skirts can give higher velocities with some setups. Hobbies worth a try too for combo of light weight and tightish skirt.
Between those 5, I'd be very surprised if you managed to find anything else that could take it over - and as a bonus they are all accurate, decent pellets, especially the JSBs, which are highly likely to be the most accurate too..
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.