I've touched on this in the past when I cracked the stock mirror on the far right. I was lazy and only replaced the bad one even though they came in a 3-pack.
A month or so ago the power on my K40 seemed a little suspect so I cleaned the mirrors. That's the best I could do on the center mirror. It is one of the stock mirrors that came with the unit. the mirror on the left is the one that was in the cutter head. Anyway, replacing the two remaining mirrors did up the cutting power back to normal. I will replace the stock lens in the near future. It looks like you can get a decent replacement for around $35.