And, retroactively, we could argue that the ovoid shape sliding into the ovoid hole in the display graphics of Chekov's torpedo-firing console in ST:TMP was a graphic representation of the torpedo casing.
I mean, the makers of ST:TMP probably intended this to be some sort of an aiming crosshairs or something. But in retrospect, it can just as well be interpreted as a graphic display of the loading prodecure, as the centerpiece of the graphic is a good match for the torpedo casing we later see in ST2:TWoK.
The only thing we know about a torpedo's top speed is that it is 75% greater than the speed of the ship launching it.
That's not an onscreen fact, tho, just backstage speculation.
And we know that probes launched from the torpedo tubes of a starship puttering about at impulse speed can span significant interstellar distances, suggesting the probes have the ability to independently accelerate to at least medium warp. It would be odd if the photon torpedoes would lack this capability. At least there should exist a long range variant of the torpedo that would be capable of such acceleration.
It's probably not a big feat at all to accelerate a small object like a torpedo to very high warp. They did it with K'Ehleyr's traveling coffin in "The Emissary", apparently without the help of a starship because otherwise K'Ehleyr would have been traveling aboard said starship. We might infer that the torpedo launchers of a starbase (say, like those on DS9) can spit out torpedoes so that they ultimately attain speeds up to warp 9 and can travel interstellar distances.
I'd like to think that torpedoes as such are fairly standardized, and most of them indeed look like that coffin prop that persists from the TOS movies to the DS9 and VOY episodes. However, it is probably possible to clip on a "booster" to enable the sort of speeds and ranges we saw with K'Ehleyr's ride. And it might be possible to fire "half-rounds", things half the length of the standard prop. Just because we don't see such things explicitly doesn't mean much, since we very seldom get to see into the innards of a torpedo launcher.
Timo Saloniemi
Some strange foresight by Starfleet maybe? 
