Anything above 135 is good.
Unfortunately, on a Corvette, once you get close to or at 140, we're only getting lower double digit increases in speed (from 135).
That really isn't going to make a difference in running away or catching most any ship in the game, just makes you feel better.
For a god roll, it's less than a 5% chance to get a plus 135 from what I've found, and the 140? Probably 1%, so bring your rolls or get lucky (based on a 50 roll trial I did, got a 137).
And the optimal mass debuff, usually when you roll, the mass and the multiplier results seesaw, so getting them both going in the right direction is even a smaller chance.
Isn't RNG fun?
On a side note, the upcoming upgrades are supposed to change the roll results with each roll better than the previous one. That being said, I doubt we will see Corvettes doing 600, FD will probably put a limit on top end for each ship and once you reach it, that's it. (Just a guess, but likely).