I've owned (in order) Wingmaster, Express, and Police...and have also read about the Police models undergoing additional checks/QC inspection. The most I EVER spent was on the Police, bought new from a CGN sponsor as I wanted "the best". The gun arrived with scratches, and the rear sights cocked about 10 degrees. Couldn't believe it. Swung by my LGS, they had 3 x 870 Express/slug guns in the back. Since I was the only customer there, I asked if I could see all three...and sight down the barrels. Sights on all 3 were perfect. Called the dealer, who contacted Gravel and eventually I had a new gun.
Between that experience, and, having a gun parkerized and learning what real parkerizing looks like in the flesh, I'm left with the impression than the 870Ps are not worth the asking price. Great guns? Sure, but no better than a Wingmaster, and neither are worth 2X what an Express is. They are much better, but just not 2X better if you ask me. I've worked on numerous Express guns now, still own one, and have shot Remington shotguns almost non-stop for close to 33 years...so I don't need anyone telling me "Express" are garbage, crappy finishes, etc. The economy has to come from somewhere, but with an ounce of care they run really well. Heck, my nephew's gun doesn't even get the ounce of care and it runs perfectly. lol You can upgrade the extractor and trigger group to bring these more in line with the WM/P models but beyond that, if you're not going to war..most people would be well served by an Express.
If there is "metal" QUALITY (not metal finish) difference between these 3 models, I'm not aware of it. I think anyone examining all 3 side by side would probably notice the roughness of the Express finish, but I've had mine over 10 years, and not a single spec of corrosion. The only time I've ever heard discussions about barrel wall/thickness had to do with Mossbergs. 590A1 vs. 500 There may be a difference with 870s, but I've never noticed.
Now if we're comparing any 870 to a DA Grizzly, I don't know what they do with those Grizzies to make them so heavy. lol Never bothered to compare one side-by-side with an 870, but I've had a couple of them over the years.
There are guys like 3Mac1 (I think I have the username right) on this forum who have probably forgotten more about 870s than I'll ever know. I've had great experiences shooting all 3 levels of guns, and have never had an issue with any of them. I suspect that has to do with giving them all the same level of care after shooting.