There is not a linear correlation between heel and speed. Eventually, you are heeled over so far, that the keel is not doing it's job, and all you're doing is sliding sideways and going slow. It looks exciting, but that's about it.
Keep it between 10-25 degrees. You can buy an inclinometer at West Marine or Defender.com.
S/V Old Shoes
1973 Pearson 30 #255