10 sens @ 12K = 20 sens @ 6K = 30 sens @ 4K = 40 sens @ 3K. They're all the same.
The reason high DPI mice are recommended is simply because they use more modern, better sensors than most mice. The reason max DPI is recommended is to pull people's mindsets away from really low DPI values like 400 and 800 typically seen on PC, because those DPI settings tend to cause interpolation and pixel-skipping on a Xim.
All you need for optimal performance on a Xim is a mouse capable of around 3000 DPI with a decent sensor under the hood. If you've got an older mouse with a 3310 Pixart optical sensor running at 3200 DPI (e.g most Zowie mice), there's no need to replace it with a 3360 Pixart sensor running at 12K (most Logitech mice), or a 3389 Pixart sensor running at 16K (new Razer mice).
The performance difference on Pixart sensors from 3200 to 16000 DPI is negligible and if anything, you're adding jitter into the mix by going with ultra-high DPI.
If you want to play it safe and go for a "one size fits all" mouse setting, I'd go with 3200 DPI and 500Hz polling, as long as you're using a mouse with a Pixart 3310, 3360, 3366 or 3389 sensor, which is most gaming mice from the last few years.
Steer clear of laser sensors whenever possible, especially Razer mice with "3G" or "4G" laser sensors. They are almost without fail complete crap on a Xim.