I don't have Westone IEM's, I have UE-11's. I also own both the Lisa and P-51.
To the op's question of "issues" with IEM's and the Lisa, no, it works perfectly. The volume control will be quite sensitive, but nothing really important. There is absolutely zero, none, nada hiss.
Comparing the P-51 and the Lisa, if sq is your only concern, get the Lisa. The P-51 is very, very good, maybe the second best amp I've heard with the UE-11's, the Lisa is a couple steps better. The P-51 gets lots of well justified praise for its mids, the Lisa is every bit as good, possibly better. The highs from the Lisa are the very best in the world, best by a pretty significant degree.
No disrespect of the P-51, hey, I own one! The Lisa is better, period.