Yes, Kelvin is constantly being discussed and people constantly waste time and energy discussing it. The truth is not democratic. Just because people constantly discuss something it does not automatically warrant importance. Has it occurred to you that the Paulo Soares has misdiagnosed his problems, has assumed that Kelvin was an issue when his problems were caused by poor CO2?
Instead of asking someone who has failed when using a certain Kelvin value you should instead ask someone who has succeeded using a Kelvin value.
Here are some essential data:
Plants grown using 10,000 Kelvin:
Plants grown with 8000 Kelvin:
Plants grown with 6000 Kelvin:
Plants grown with 4000 Kelvin:
So at the end of the day, the hobbyist must be able to grow problem free plants, full stop. Only then can he/she even hope to investigate the root cause of problems. I can assure you that if you have problems in your tank using 9000K bulbs, the reasons CANNOT be attributed to 9000 Kelvin. Conversely, if the plants grow healthy and problem free, neither can the success be attributed to using 9000 Kelving because there are many more important factors in plant health than what color bulb you are using.
Cheers,