The massive variation in human intelligence and the positive correlation between IQ and pretty much everything good implies that "Any simple major enhancement to human intelligence is a net evolutionary disadvantage" isn't true

There's also the saying that "correlation does not imply causation". The brain is very complex and energy intensive; basically anything that messes much with you is going to mess with your brain. For example, a near universal symptom of genetic diseases is reduced intelligence -- and I'm going to bet that low intelligence is not the cause of the genetic problem.