I don't know if they sell them after-market, but my Thirty-Two Focus boots came with a set of adhesive pads that you stick to the inner boot to tighten up the fit...they work great. I mean, really big difference. As others have said, heel lift not only changes your ability to make quick turns, it also can really, really hurt the front of your foot as you're constantly trying to compensate. Your heel/calf should be doing the work. If you're having to constantly put pressure on the balls of your feet to compensate for heel lift, I guarantee you'll be a candidate for surgery! Been there, done that. Ripped the shit out of my metatarsal tendon. When I went to the doc I just said "snowboarding" and he knew exactly what was going on.
On this same subject, going slow is a BIG problem! My skiing friends don't get it..."Let's do a green, it will be mellow." Fuck that...greens are THE WORST. Trying to hold an edge or maintain any kind of speed on a flat green is the worst thing for your feet. You're putting lots and lots of pressure on the front of your feet just to maintain...it not only hurts, it rips the tendons in your feet. Trust me...after thousands of dollars of surgery and an entire season out, you don't want to go through it.
Invest in good boots, have them fitted!