boosters are wicked. put them under your shell, directly on your liner and they will get rid of any place you may have in your upper boot. with out the slightest bit of play, you wont be able to bang your shins on your boots because they always move together. you can do this with a normal power strap, but its not as nice. the booster is elasticized so it gives you a progressive flex. it works wonders for park because its squishy, and basically most big mountain rippers use them becuase it gives a much tighter more personal fit.
your shop shouldnt charge you to install them if you buy them there.
to stiffen up your boots, to the guy who asked, just stick a bolt through the upper cuff and lower cuff on the calf. this has to be done higher then the original attaching point. all it does is take away leverage on your boot.