I also bottomed it with original setting.
Than I bought Hyperpro progressive forks from Calsport
http://calsport.co.uk/yamaha-xtz-660...ro-suspension/

I use 12,5 fork oil.
Also bought Excel rim and stronger spokes.

Bike is now behaving much better.
No more bottoms, eats well bumps and potholes. More stable also.

If you wanna go serious, than you need to do YZ or similar front end conversion.