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I'll have to do some more research, but it seems like it should work with GPU rendering as well, even multiple. Might be fun running it on some older mining rig.
This extension should allow it, but I haven't tested it, yet.
It should reduce rendering times significantly, if you manage to make it work :)

https://bitbucket.org/sbrodhead/faengine

Just saw that there was a kickstarter project to add GPU-rendering to JWildFire as well, but it did not reach its goal.