The best way to stiffen an spk is drill a hole about 2.5cm above the rivots in the cuff. You cant easily do it in the center of the cuff as it already has a v cut out of the upper clog. If you use a t bolt and a screw you can alter the flex for differnt skiing.
One nerdy is right about matching flew to the leg, but I still lilke a stiffer boot for big mountain and softer for park. Being able to change the flex on the hill is awsome.
With 2 extra bolts I got mine as stiff as a 130 ghost. I also tried adding a second cuff buckle but that stiffened the boot to much for my liking.
Any good boot fitter should be able to do the mods.