One thing I've noticed having partially disassembled my P12's keybed is that they are very sensitive to two things: the position and lubrication of the rubber bumpers at the front of the key guide. There are two rubber "bumpers" which sit on pins towards the front of the key support - the key has a plastic loop that fits over these bumpers which then provide vertical and horizontal stability as the key is depressed - keeping it from wiggling side to side and ensuring it doesn't twist on the way down. On my keyboard, some of the bumpers seem to have a "thinner" and "fatter" side, and one of them was even installed wrong, causing a fold which increased it's thickness enough to cause extra drag on the key. This is one potential cause for unevenness. Obviously if the bumpers are not well lubricated then the key will also drag on them slightly depending on how you play - this can also result in a "slow" or "draggy" key feeling or prevent a rapid, crisp return. I am not sure which lubricant to use, some people use a Dow mold release lubricant which is horrifically expensive, others use a Yamaha keyboard-specific lube, etc.
Velocity-sensing problems are a totally different story, though, and I expect those have more to do with the problematic graphite contacts and "rubber dot strips" midway back on the keybed, as they do with Fatar actions that occasionally run into similar issues.
I agree that this keybed does seem horribly inferior to nearly all others, but hopefully you can get it regulated enough to enjoy playing it with some simple efforts. Let us know how you get on with it!