I have been pondering this for a while.
Anytime you butter, you are bending the ski (obviously)
Anytime your boot flexes, this is power that is not being transmitted to the ski. Thats why racers are in such stiff boots.
It seems to me that the stiffest possible boot (with in reason) will allow you to transmit the most force to the ski allowing you to butter easier.
A large part of this trick is being able to get over your ski and maintain a proper center of gravity throughout it. So a boot that is to stiff will not allow you to do this.
I personally ski the lange rx130 lv with a worldcup booster. While it is by no means the stiffest, I feel like it is pretty stiff compared to my other park rat friends.
I personally believe that getting into a stiff boot has made my park skiing a lot more consistent.
what do yall like and why?