Not the OP, but the way the rocket equation works is that for the same ratio of gross to net mass if you double the exhaust velocity, you double the delta-v. So

Not the OP, but the way the rocket equation works is that for the same ratio of gross to net mass if you double the exhaust velocity, you double the delta-v. So, it's exactly linear. It's actually a little sublinear, as you add the delta -v to the original v (which is the speed of the planet Earth plus the orbital speed of the vehicle in the LEO that starts the trip to Mars). So, if you double the delta-v, you don't actually double the deep space velocity. Where the rocket equation helps is in the net to gross mass ratio. Starship is expected to have a 9% ratio. If you multiply the ISP by 2.2, you would increase the ratio to 34% (as someone else in this thread mentioned). The Starship proposed net mass is 120 tons, that could go to 450 tons. For comparison the largest (cargo) airplane in the world, the Antonov AN-225 has a maximum takeoff weight of 640T, but 300 of that is fuel.