I've used the lye from the hardware store and food grade as well. Couldn't tell a difference.
If you're worried about the difference in purity, it is likely not very different at all.
Most lye is make by the "chloroalkali" process which start with salt and water and runs an electric current through it. It is very unlikely that they would use rock salt in their reactions (that could be contaminated with heavy metals) because then you could create different side reactions which could present some problems during production. That means they probably use refined salt which is already low in heavy metal contaminants. So, in this case, tech grade and food grade are probably very close to one another. And probably differ in the handling after production or the place of manufacturers' certification.
I know that for a lot of lab grade stuff, the difference between one grade and another is actually the testing and certification done on it. Its possible that's some of whats happening with the lye. Of course, I'll never know for certain and this is just speculation.