If you do want to try to supercharge it, the post-2005 Turkish Tempests were widely regarded as over-sprung and hard to cock. Chambers list the same replacement spring for them as for the proper Webley ones (which I think is made for Chambers, not new old stock). It’s possible that Highland Outdoors (who own the Webley brand name) might be able to source one for you.

But I agree with Baz and hwtyger: don’t do it. It will make it all hard to cock, slammy and boingy and stress the cocking mechanism.