i would like to direct you towards Jeremy Clarkson's view in Top Gear magazine a few months ago that faster cars are safer. to paraphrase:
faster cars have bigger brakes, and seeings as all the cars in your trip were travelling at the same speed, chances are the fastest car you saw (if it wasn't your own) is likely to have had the shortest braking distance. similarly, faster cars have better suspension. this means they have higher limits than slower cars, and that means you have to be an utter tool to crash them.
based on this, the more horsepower the better, and there is no limit on this
tbh the reasons you've highlighted are the reasons why i've chosen to upgrade other elements of my car rather than getting it remapped. you can't utilise the performance, so what's the point? in any event 0 - 62.5mph in 6.9 seconds (the time quoted for my A3 2.0T DSG) is plenty. yes, the chassis may be able to handle more, but the simple fact is that straight line performance wasn't it's weak spot. things like body roll and braking were. so to me these are what i'd like to improve, not the power.