[Zeke]

[NOTE: Jedi snuck in while I was typing this.]

Short answer: There's probably a gap.

Long answer: I don't think there's any canon evidence about how fast full impulse is, but Warp 1 is traditionally the speed of light. Full impulse would have to be considerably slower than that, because if you're not warping space, relativity applies to you -- in particular, you experience time dilation based on your speed. A day's travel at half the speed of light is 1.15 days to the rest of the universe. At 99% lightspeed, that day becomes a week. It would be impossible to coordinate a fleet of ships with that sort of time dilation going on, so full impulse couldn't be very close to the speed of light.

Of course, Trip and Malcolm were out of options, and skipping ahead a few years is better than dying... but that's where the other consequence of relativity comes in. The more you accelerate, the greater your inertial mass, meaning more energy is required to accelerate you further. Accelerating to the speed of light is impossible; accelerating close to it takes a prohibitive amount of energy. In conclusion, never get into Shuttlepod One.

Incidentally, there was an Andromeda episode that made clever use of this idea. Before slipstream propulsion was discovered, Earth built a ship, the Bellerophon, that could actually meet the insane energy requirements of near-lightspeed, and sent it out to explore. The crew knew the universe would change tremendously in their absence, but felt it was worth it. So they were still out there after Dylan's 300-year nap and ended up rescuing him when his slipstream drive was destroyed, which left him with a dilemma: stay where he was on the slim chance the Andromeda could find him, or let the Bellerophon take him to the nearest planet and lose another 57 years of relative time? Cool episode. Tony Todd was in it.