So this would be the ONI titan? or is this the corvette? if its the corvette i have a idea for the titan.
It shouldn't be the titan.
A Laser would be pretty cool thought there would be some pretty big technical details. A simple up-scale wouldn't suffice, the bigger the laser the bigger the power needed to get that thing going. It would be a case of economics, 'does this single laser blast do more damage than a MAC round?'
The SpLaser ship would fit better into the post-war era when the UNSC finally cracked the problem of large scale implementation of shield tech. Shields require a lot of power to run, perhaps the SpLaser ship could repurpose the shield generator for the laser instead. This would create a super strong glass cannon, and the lack of shields would simply pe explained away by saying the generator can't power the shields and a laser at once -we're good, but not that good yet- kind of argument.
It requires lots of power for shields, lots of power for plasma, lots of power to make the interior of a ship entirely out of hard light. Things similar to this have already been done in the Halo universe, just never used by the UNSC. It would have to be post war so the UNSC would have at least some understanding. I think it should be ONI specific where as MIC get the Orion.