I'm actually more worried about CPU performance than I am about corvette performance. Though it might not be feasible for one player to build 2000 corvettes, if every player in a game with 10 players builds 200 corvettes, you still have 2000 corvettes running around.
Since multi-threading isn't an option, here's to hoping that the developers are working on some kind of rendering shorthand for corvettes. I don't care if they render the things as BRICKS until you zoom all the way in, so long as the game runs smoothly in large games.
Alternatively: when Rebellion sells well, take some of that money and devote it to implementing multi-threading.
With regards to balance:
The idea with corvettes is that they are fast glass cannons: high DPS/cost, good DPS against all armor types, but low individual DPS (so the "critical mass" required for a corvette ball to be unkillable is relatively high, as opposed to a LRF ball, which has a low "critical mass" threshold since individual unit DPS is so high).
But if you make corvette health/cost too high, they'll outmode light frigates completely (as it is, they're already on their way out). So the model we're looking at here is as follows:
Corvette: high DPS/cost, low health/cost, low DPS/unit
Light Frigate/heavy cruiser: low DPS/cost, high health/cost, low DPS/unit
LRF: high DPS/cost, low health/cost, high DPS/unit