Engines =/= Reactors. Reactors feed power to the ship weaponry/subsystems AND its engines. The engines draw on power from the reactors to propel the ship forward/activate subspace.
On another note, Vasudans have better beams than their Terran counterparts (SVas is superior to the SRed and SGreen, VSlash is 2x as good as TerSlash, BVas is in between the LRed and BGreen in terms of sustained damage output). Not to mention, they make reactors only beatable by Shivan design.
Now, just think of the power a Vasudan/Shivan reactor could produce. Take a look at the Lilith and you can see how insane that would be (a small cruiser with durability comparable to a corvette, decent top speed and a LRed for capital ship killing). If they made those kinds of reactors for the Colossus, it wouldn't need to overdrive its reactors for 'High Noon' since it could support LRBGreens as standard firepower (I would've preferred to fit LRBVas since the Vasudans have better beam weapons as I mentioned, but the heatsink issue still exists, though if you use Shivan materials that could be solved).