Remember the time ship Aeon.Yes. Everything was rushed. Leaving dock early, less than a full crew aboard, Kirk ordering full impulse ("Ahead warp... point-five."). And you left out some important in between data. 1.6 hours later they were passing Jupiter and Kirk recorded a log entry saying that to get the earliest intercept point with the Bogey he was going to risk engaging the unbalanced warp engines while still within the Solar system. Sulu accelerated up to near the threshold (which takes enormous energy), presumably to minimize transition shock. But they were at warp one when the wormhole formed. After they dropped back sublight, they had lost a little kinetic energy, but were still "coasting" at 0.8c -- near what they'd been traveling at when they entered warp. The high sublight speeds were both for the emergency situation and due to uncertainty about the warp drive.
The TNG Tech Manual says that the .25c "speed limit" is for normal operations, to minimize disorientation and having to constantly readjust both clocks and perceptions. Usually, a ship warps in close to their destination and is fine maneuvering for a few minutes or hours at those fractional-light speeds. Emergencies and combat? Do what ya gotta. If you're dead, it's not going to matter that the ship's chronometers are out of sync.
https://memory-alpha.fandom.com/wiki/Aeon
It had "Hyper-Impulse" drives mentioned on screen. In my Head Canon, I think the "Hyper-Impulse" was a modified Impulse Drive System that allowed normal ships to approach 1.0c or close to it without having any of the negative effects like "Time Dilation" and constant resetting of clocks, all without going into "Full Warp" and using the "Warp Drive".
I think it was a cut-back system that created some form of Static Warp Field & other SubSpace field that lowered the Inertial Mass of the vessel even more than normal Impulse would, and use less power than the traditional Warp Nacelles to accomplish the same task. This would allow you to cross the 0.25c barrier and closer to 1.0c without actually getting to 1.0.
We know that as you approach 1.0c due to E=MC^2 law, that the Energy output for going ever closer to 1.0c would rise exponentially, that energy cost would probably intersect the point where it's more energy efficient to activate warp drive and go >= 1.0c. So that limit acts as a logical transition system where the computer would be programmed to use the more efficient propulsion system.
That would mean activating the Warp Drive as needed to not waste energy on going faster in SubLight beyond that intersection point when it would cost more energy to go faster at SubLight speeds. That intersection point would be crucial and be defined in the tech specs of most vessels with Hyper-Impulse systems.
And I believe that upper StL(Slower than Light) limit would vary from vessel to vessel based on Volume/Mass/Shape/etc, a whole multitude of factors that would affect it's Hyper-Impulse upper limit before it makes more sense just to hit Warp Drive and go >= 1.0c.