Hi,
If you open up a PanWorld pump you will notice that there is no tolerance for your impeller to go anywhere. The impeller seats all the way against its housing on the bottom, and then you have a
"valute" (sp?) that hold the impeller in place on the front side which makes contact with it. There is absolutely no where it can go, be it forwards, backwards, up, down, left, right, etc... Even if it did slight shift forward, it would be entire wet end housing, which makes no difference at all.
I know of a couple people who have both PanWorld's and Iwaki's installed this way with absolutely no problems to date. HTH!
Thanks,
Jeremy