Most wheel bolts have a bulk hardness between 32-35 Rockwell "C"....... BUT, most of them are cased with a thin (0.003") case with at least 77 HR15N (don't have the conversion at my fingertips). So, even if you succeed in cutting threads farther up the shank, you'll lose the benefit of the hardened case, which imparts a lot of fatigue resistance. The fatigue resistance is not just from the extra hardness, but from the residual compressive stresses in the case. Plus, all the wheel bolts I'm familiar with have a corrosion-resistant coating. The cut threads would not.
The bolts are purposely designed with tight metallurgy, and are a critical safety component on the vehicle. They are so critical that the Transportation Ministry of Canada has a zero tolerance policy on defective wheel bolts. I urge you not to mess with them. There are many different size wheel bolts available on the market. Try to find a size that fits your project.
William, hardness has a nearly one-for-one correlation with strength. But as you can see from the above information (and as you suggested later) hardness isn't the only criterion for a part that won't fail during use.
95 E320 Cabriolet, 131K