The higher tier wormhole techs list this as a modifier: "ship_ftl_wormhole_speed_mult = 0.25
The wormhole drive itself doesn't upgrade, so this seems like it must be an empire modifier or a modifier to the stations. But what would be likely to happen if you put it on a modified wormhole drive component? Or would it be better to use the aura modifier, "ship_windup_mult"?
Edit: Something odd I noticed is that if I have a faster "version" of a kind of FTL drive (in this case, a hyperdrive), then mixed fleets will traverse hyperlanes at the fastest speed. To clarify what I mean, I'm working on a kind of quick light scout vessel. These can mount all the normal FTL drives, but also faster, specialized "recon" versions that let them move between stars more quickly. I had expected that scout ships built with "recon" drives wouldn't be able to join fleets of ships with the normal version, but they do. Further, the entire fleet benefits from the recon speed enhancements. Why is this? I suspect the game determines what counts as a "different drive" according to the tech prereq.