^ Right, we leave non canon speculation out of it and then this thread would consist of people going back and forth stating ONLY what the EXACTLY saw on screen, and they wouldn't be able to make any leaps or connections that weren't specifically spoken on screen.

Besides, Memory Alpha only uses canon sources to back up everything they say, Memory Beta is the one that uses non canon sources, so anything Memory Alpha says has as much chance of being accurate as anything we say here if we only go by canon evidence. Anyways...
The Phoenix. If we are to go by only canon evidence, what we see and hear on screen, then all you have to do is LOOK at the shot of the Phoenix ANY time before it HITS warp speed and you can clearly see that it is traveling at a constant speed. From the moment it leaves Earth's atmosphere until the moment it achieves warp speed, it is moving rather slowly. Slow enough that the initial rocket could have caused that momentum. Now, we know that the warp drive bends subspace around the ship, creating a bubble which envelops the ship and distorts the space-time continuum, and propels it at light speed and beyond. Well, I propose that the reason Riker said that they were approaching light speed when they were clearly not moving any faster than before, is because he was talking about the time to lightspeed, not the acceleration left till light speed. It would make sense that an older and prototype warp engine might take longer to build up enough of a distortion in subspace before it had built up enough to push the ship into warp. Riker could easily have been referring to the engines having built up almost enough of a distortion to propel them to light speed, and was announcing that they were approaching the moment of light speed. How would you word that other than "approaching light speed"?
As for the Romulans of Kirk's time, It is not mentioned whether that is indeed a deep space vessel or not. It could easily have been a short range vessel launched from an area near the neutral zone, as they were not far from the neutral zone during their encounter. It could just as easily be that perhaps Impulse drive can get you above light speed, although it'd most likely be at some cost of being fuel inefficient or not as fast or Warp Drive, or something otherwise why wouldn't Starfleet use it for light speed? That being said, who's to say it uses the warp nacelles? Why would it have to? Do we even know if the two systems are compatible that way? As for the evidence of STXI's Kelvin, I wouldn't put it past them to have done that just to look "cool". And I loved that movie. I just think that connecting the two with so little on screen evidence and not even any dialogue to back it up is way too much of a stretch.
And let's not forget that the
Impulse Reactor is the component that provides power to the
Impulse Engine. If Impulse uses the warp nacelles for propulsion, then the power would need only move from the Impulse Reactor to the Nacelles... what use would another engine or drive system be if not for the actual propulsion? It really is just too much to say that the Impulse engine uses the warp nacelles for propulsion. The only thing that really points towards that is the one scene with the Kelvin. The whole Impulse in reverse can be explained by the use of subspace fields generated by the impulse engines themselves, as Memory Alpha suggests they have in addition to the rocket-based propulsive effect. It could also be explained by smaller impulse engines being located at various other points around the ship, as was questioned in the original topic.
I also find it very telling that the Impulse engines are always located in great areas to provide excellent thrust for forward motion.