Yes, you can definitely mill too fast with a drill and get excessive husk pulverization which can
potentially lead to "off" characteristics in your brew.
I had been using my 1/2" variable speed drill for several batches and would have to simply use finger pressure to go as slow as I could, which I
thought was slow enough. The crush was good; I increased about 5% efficiency from my lhbs; but the husk was more broken than my lhbs. I DID seems to get more grain coming through my false bottom too. No big deal and the beer came it good.
I decided to buy a dedicated 1/2" drill from Harbor Freight so I don't burn out my other drill. The new drill has a speed adjustment dial so I don't have to use pressure to find the sweet spot. After my first use, I realized how much better it was than my older drill. I was able to dial down to a very low RPM and simply pulled the trigger. It went slower than I was ever able to get my other drill to go. The crush was equally as well, and I got less flour and the husks were more intact.
Overall, I noticed a significant difference between the two drills and I can honestly say I did not expect to. I was just trying to save my other drill for work around the house.
The harbor freight drill is this one:
http://www.harborfreight.com/1-2-half-inch-heavy-duty-spade-handle-drill-93632.html
and I used the 25% off coupon for a total of about $38. The handles are removable and can be placed on all sides.