The strap on the SPK's is better than most other boots straps. I'm sure booster straps would make some improvement, but I'm not sure it would be worth it on SPK's. If you're having shin bang problems don't land on your ass.
I put a set on my foils. Ive only been out twice on them, and they are so much better than the stock power strap. They never come loose, and flex great with your leg.
I had shin bang problems with my spk's....if your over 160 lbs then the spk is most likely to soft for you. look for a stiffer boot...perhaps one of the stiffer spks?