i would never pick a ferrari, just not my style. bit too highly strung and over tuned, its the sound for me, and they don;t do anything for me. but the lazier tune and less high reving engines of the likes of lambos and astons i do appreciate. if i had the cash i'd be looking at astons or maserati's or even new f type jag for a daily driver. wouldn't say no to a gayardo. but if i had a track toy it would be light and nimble. like a caterham, elise, xbow or kit car.
i reckon the 911 sits nicly between the 2 categories, not too light, and not to heavy, plenty of punch, lots of feedback. probably put the M3 in the same box as well.
I found when pushing on with the zed that its weight became the disadvantage once you got passed a certain point. it hampers the car. great for novices to RWD as it makes the car very predictable and controllable in oversteer, but annoying when your really trying to push it.
so with regards to your question whats the right amount of power to weight, i give my 2 examples above, a 911 and an M3. yes you can get more power, or lighter cars, or more torque, or better steering response. but those 2 cars i would say are about right.