I'm inclined to think it best to go for the whole hog, so to speak. Your proposed changes aren't game-breaking or unbalancing. A smaller step, while an improvement, may prevent later improvement. "Why do you need this? We already gave you that."
A five-charge Tech Override would be an improvement, but it would still leave us sub-par to other classes. We'd be competitive for a single rotation and then be easy kills for the rest of the encounter. With a five-charge Tech Override, though, the arguments of being over-powered with instant cast unload and such would start rolling in. If we could cast any of our harder-hitting abilities instantly people would start screaming that we're over-powered and calling for us to be nerfed.
Some people are set against the idea of a permanently instant cast Tracer Missile/ Power Shot. But isn't that better for balance than giving mercs the opportunity to instant cast any of our stronger abilities? Imagine this rotation...
Unload, Tech Override, Tracer Missile (three times), Rail Shot, Vent Heat, Fusion Missile, Unload, Heatseeker Missiles.
None of which can be interrupted. Vent heat keeps us going and since it's already an instant cast ability it doesn't use a charge of Tech Override. That rotation could dish out 8662-9514 unstoppable damage (That includes the bonus 1944 DoT from Fusion Missile). That would destroy a sizable chunk of player HP. That full rotation, if used, would take us right to the edge of our heat, but we would be instruments of pure carnage.
How fast would players start calling for a nerf? We could be devastating (If we don't die before finishing the rotation) from the edges of a fight. With proper target selection and LoS, we could finish off targets with relative ease. Mercs would have a glorious run for a short while and then feel the weight of the nerf bat.
It's safer and better-balanced to make Tracer Missile and Power Shot instant cast.
True. I also just realized that a Power Surge like that would be usable by a Combat Medic Merc. THAT would create imbalance; 5 instant rapid scans every 60 seconds would definitely increase healing output significantly, which is not my intention.
We really only want Tracer Missile and Power Shot to be able to be used instantly. There would be no change in use when not under pressure (cast time = GCD), and survivability would go up because we could actually defend ourselves when the time came, rather than just hope to run away.
Unload is Channeled tho, so would still be able to be interrupted, even with Power Surge which only affects abilities with cast times.