The really big change will be launch thrust to weight ratio. Going from ~1.2 to ~1.35 gives you 75% more thrust at launch which means you spend less time fighting gravity, less time in the thick parts of the atmosphere, and less time to get past the trans-sonic region.
There are other constraints on how quick the vehicle should be, even when engine performance allows: you probably won't want to hit maximum dynamic pressure in too-thick air.
