I dont know about it being a 'good idea' for any of us to own one of these beast. Chances are adrenaline will overpower the brain at some point and you will find yourself compromised. Seriously it sounds to me, by your very question that you respect the power the k5 has and that is a good start. Never disrespect the power or you will end up eating it. I would not detune it personally, that is what makes it the bike that we all covet. The k5 is really pretty easy to ride slow on, it has a ton of torque. You may not be able to just rip on the throttle. You must develop throttle control that you dont need on the smaller machines. Also speed is very deceptive on the k5. It will go faster in 4th gear than most motocross bikes will do in high gear. Top speed depending on gearing is around 100mph. My experience has been that a stock Cr500 will not touch a stock Kx500. I actually leaned that way as well, but then my brother in law corrected me and now I love that k5. Also the CR has a much newer design to it. The k5's have not changed since the mid 90's and really very little before that as I understand it.
I also have a KTM 300 MXC, and as anyone who has both will tell ya, the k5 in a drag race overpowers the 300, but once you hit the technical stuff the 300 will walk away from the k5. The ktm 300 is the best all around bike period!!!! Great power, and awesome suspension, handling, ect... The KX 500 is all about pure adrenaline. As far as reliability the KX 500 is very reliable. It will eat your chain though, but we call that maintnence.