Actually it's a misreading of the TNG Tech Manual. What the manual says is that, due to relativistic considerations, "normal impulse operations are limited to a velocity of 0.25c" (p. 78). On p. 75, it defines "high impulse" to mean "velocities above 0.75c." At no point does it use the term "full impulse" or equate it with any specific velocity.
So the idea that impulse engines can't go above 0.25c is pure urban myth.
Of course, there's no friction in space, so continuous thrust would result in continuous acceleration, not constant speed. So there wouldn't be any "maximum speed" a ship was physically capable of achieving. Heck, a lightsail accelerating at 0.0001g will eventually reach a fair clip of lightspeed as long as it can keep accelerating long enough. So "full impulse" would refer to a maximum acceleration, not a maximum velocity.
