"Unsafe" is so relative and dependent on the driver/driving environment as to be pretty difficult for anyone to offer you an answer that will really help!
EG: I'm quite happy with a car that does 0-60 in 15 seconds. Are you?
Keeping the engine in lean burn mode is key to saving fuel on the highway, as you know. If you load the engine up so much (through high speed or other high load condition - eg. possibly too much weight for the engine in a hilly area), there's not much point in doing the swap. How fast do you want to cruise on the freeway?
A VX owner who has driven with 2 big passengers on board could probably answer your question about power:weight effect on holding lean burn.
Aero mods will help reduce freeway engine load too, of course.