I think ships should have different top speeds, based on mass.
I also feel acceleration differances should be doubled.
From fastest to slowest, a raw example.

Light Fighters
Med Fighters, Light Cargo Bombers
Heavey Fighters, Med Cargo/Bombers
Heavy Cargo/Bombers/Proms
Frigates
Destroyers
Carriers/Light BattleCruisers
Battle Cruisers

I am no phyisicist - I am a gamer. If you need a roleplay reason to make ships go different speeds, just make it up.
"The mass of the ship is in direct correlation to the graviton field utilization atrophy according to the skinko equation in the engines of modern spacecraft." - or whatever floats your boat.

The accelerations are the only things that are different, and for balance I think this works very well. Also if you made the differances much greater the valk would be completely broken. I think it's perfect as is.