The whole thing regarding various "time barrier" technological hurdles has been bandied about and there are of course various theories. I like to think that warp drive technology does have natural stages of development, which may relate to various factors: reactor power output (I would not expect a standard-spec matter/antimatter reactor of an NX-class vehicle to have the same magnitude of output or efficiency as a Constitution-class vessel), red-line/yellow-line of the "engine" (I would imagine that early warp engines had less capacity for fuel intake than later designs), nacelle stress (an NX class nacelle may hande Warp 5 in a completely different way than a Constitution-class nacelle), as well as overall ship's hull stress (remember Kirk's warning to Nomad when Nomad pushed the Enterprise to Warp 11). These are probably just a subset of the issues that affected the evolution of starships and warp engine technology.
I would expect that the NX Project was eager to play "catch up" with the Vulcans by learning the space-warp "tricks of the trade" that balance the above concerns to achieve better performance. That's probably why refits and newer starship classes with more streamlined designs are so important.