Please do send all events (including meta events). Perhaps you don't need them with your Midi files, but I have work with Midi segments that changes tempo (perhaps even time signature 3/4 <-> 4/4) on the fly. So looping back will need the meta events (tempo...) to play properly. There are other meta events, too.
Note that FluidSynth's player automatically handles SET_TEMPO events (even though it doesn't forward them), so you shouldn't need to handle them manually. But I take your point.
If you think the extra "setup time" may take too long for your usage, perhaps an option to specify whether to play all events, or allow omitting of certain event type(s).
I doubt there is any performance consideration here. This proposal would change meta-events so they are handled the same as channel events, and since a typical MIDI file has vastly more channel events than meta-events, that probably doesn't change the performance at all.
My main concern was that some playback_callback code might not be prepared to handle meta-events, and could break once we start sending them. But since playback_callback is (AFAIK) new this year, I doubt that will cause much trouble, and it will be better to fix this now rather than later.
So yes, I agree, I think it should be changed to pass meta-events down into playback_callback, which can ignore them if it wants to, just like the default one does.
Matt