In the TNG Episode The Emissary, we learn that a Class 8 probe can travel at warp 9 (which puts quite a strain on a Galaxy-class starship, let a lone a probe!). The probe in question was stripped of most of its features to fit a passenger (I shan't name the person to avoid spoiling the episode) inside, who occupied the vast majority of the probe with very little room to spare. Hence, how does the probe manage to sustain such extreme velocities and where does it source its power from?
Star Trek: The Next Generation Technical Manual
- The propulsion system of the torpedoes is a warp sustainer engine. The engine coils of the torpedo grab and hold a hand-off field from the launcher tube's sequential field induction coils. A miniature matter/antimatter fuel cell adds power to the hand-off field. When launched in warp flight, torpedo will continue to travel at warp, when launched at sublight, torpedo will travel at a high sublight speed, but will not cross the warp threshold. (pg. 129)
- Class VIII medium-range and class IX long-range multi-mission warp probes both use a modified photon torpedo casing and a warp sustainer engine as the power plant. (pg. 117,118)