not likely to be a great idea..
,177 already causes high pressure in springers (.20 is better ). A heavier pellet will cause even higher pressures, although the JSB is somewhat soft lead which helps with start pressures.. but still..
High pressure means the piston bounces sooner in the firing stroke, reducing efficiency.
No harm trying a few to see how they shoot (and chrono), but I'd set your expectations low.
You can always sell the JSB heavies
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.