Be aware there a two different Hashboards actually.

https://support.bitmain.com/hc/en-us/articles/360002072833-L3-Different-Version-Comparison

Hashboard L3+ (S) is the old PCB and could handle Overclocking up to 600 MHs
Hashboard L3+ (N) is the new PCB and is very poor in Terms of Overclocking (around 540 Mhs only)

So be aware when overclocking newer L3+ they have changed the Boards somewhere in early 2018.