Was thinking about doing that. 9mm is good plinking ammo, and I don't own a 357 sig gun. I already own a P7M8 so I am not sure if I want the 9mm barrel. Any experiences?

I'm going to order a .357 Sig barrel from KKM once I get around to it.  As for a 9mm barrel, Novaks does a 9mm conversion, but it's more involved than simply putting special 9mm barrel in the .40 gun.  The .40 has a different slide, with 3 locking lugs instead of two.  Also, 9 has a different breechface, though the difference is minor.  Finally, you would need different mags, recoil spring, extractor, ejector, and perhaps some things I'm not thinking of now because it's late.  A .357 sig barrel swap is easy.  Going 9mm on a .40 is not a great idea, unless you want to get a new top end and ejector.  It can be done, but it's not straight forward.

I would completely forget the swap to a 9mm for the reasons hobbs listed. Life is too short to go thru all the BS it would involve to make that happen, IMO.

My .o2

Let me know how the 357 sig conversion. Not too dissapointed about the 9mm, my P7M8 is my favorite 9mm anyways and doubt I would like the Hop much more in 9mm.

Mostly got the 40 cal HP for CCW, and I want some weight in my 40. I don't notice my 30 oz (unloaded) P7 carrying, maybe amy HP w/good belt/holster it will be almost as easy.

On the rare occasions I carry a Hi-Power, which is when I'm out of my home state, I carry in a custom holster made by Mark Garrity.  Since I normally carry a Glock 21, the Hi-Power seems small and light to me.  Check out Garrity's holsters.  He has custom holsters at the price of, say, Galco holsters.   His work is top notch, and his designs very well thought out.

garritysgunleather.com/