Can't comment on the 599 or many Ferrari cars as I don't have much personal experience with them.
As for the Diablo. It's not a good daily driver if that's what you're asking, but it is nice nice when you're on a track with it, and not sme stupid quarter mile or round track lol. It's a pain in the ass shifting the gears in the city. I'd imagine any manual would be but the Lamborghini is just horrid when trying to get it off the line. It is rarely used for a leisurely drive.
As far as the upgraded horsepower. No, it isn't needed at all but that's where the desire to make it perform even better comes in. Of course it needs some suspension work redone and better brakes, but this is all for track purposes. The car itself is a pain in the ass to get into, but I'm only 5' 8" so it doesn't bug me so much. I've got strong arms so shifting it on the track isn't a problem at all. The upgrades themselves will set ya back $60,000. It's a vice!
I would love to get my hands on the LP 640 Murcielago, but I'm not willing to spend the money on it just yet. I will probably skip it and wait for the next iteration of whatever Lambo brings out.