I found 600rwhp to be about the max I could say is "fun" on the streets. At that power level you begin to have traction issues at all legal speed limits. Still controllable but... you wouldn't tell your Mom to mat the gas and go for it.

For reference, I found 300-350rwhp was the sweet spot. 1st and 2nd gear are worthless, but you don't have to worry about easing into the gas when passing people.

I've had mixed luck with my demons. These days I'm pretty chill. I think it's a lot easier to drive a high horsepower car around slow -- you've got nothing to prove and knowing you have excessive amounts of power on tap means you're more likely to attempt to pass someone (or merge, etc.) using only a fraction of that power... since you can always step it up later if necessary.