Out of boredom today, I was playing around with a typical ~$20 Schlage consumer grade entry lock. I duped the key and got one three generations down that still worked flawlessly. Started to wonder how flawed the key could be and still work.
Well, I got out a dremel and started filing down this 3rd gen key. I was pretty shocked with how far I whittled it down with it still working. I rounded out the spikes in the key and knocked every cut down by around a half a depth, then knocked the cut closest to the bow to what is approximately a #7 cut. The original bitting was 44646 and I have one that's effectively 74646 with the 4646 cuts being a little deeper than they should be (measured with a Schlage metal gauge). The key works absolutely flawlessly. It's a little alarming that one cut could be three levels deeper than it should be and the key works flawlessly.
Here's the key in that state next to the original:
Makes me wonder if Schlage's consumer quality really is much better than Kwikset. I'll have to try this with a Kwikset some day and see what the results are there.
Eventually I filed it down to a 75757 (after I took the picture) and it will still work, but it requires a little bit of determination (no, not treating it like a bump key, but it's not going to turn first try by any means). Just very interesting