Current factory spring ~ 825 fps with a 14.3 gr. CP is considered about right.
Maccari GSI kit maybe 785-800 fps with one of the several 14.3-ish pellets, and 20% less cocking effort.
Maccari Monolith spring, or a hot factory spring (more common in early 90's) could do 840-855 with a 14.3.