I picked up that same 870 Tactical a while back. I wanted to upgrade the 870 P parts as well. I did a bunch of research and discovered that the only things I really cared to upgrade was the extractor, magazine tube spring, and carrier dog spring. I also upgraded to an enlarged safety switch. I rather like the polymer trigger guard and they have actually switched to using the polymer ones for the 870 P now because they are actually stronger. Some people just dig the metal guard, and I'm cool with that. I like the metal guard on my 590A1 too.
This 870 Tactical is a pretty sweet little shotty. I still prefer the 590A1 if I had to grab one, but I do like it better than my 500(in it's stock formation). I like the 870 fore grip design. Considering ordering the Magpul stock.
Really, the only disappointment that i had with the 870 was that the receiver isn't tapped from the factory. If I had known that, I would have gotten one of the ones that had the top rail or ghost rings form the factory. I run a red dot on my 590A1 and it works very well.
Glock 19 G4 Gray | Glock 17 G4 FDE | S&W M&P 22 | S&W M&P 45 | S&W M&P 9 | S&W M&P Shield 9
Sig P226 MK25 | Sig P229 Extreme | Sig P320 F | Sig P320 C | HK USP 9 | HK USP 45 | HK USP 45 (Tan) | HK VP9
Daniel Defense DDM4 V4 | Arsenal SGL 21 | ASA SBR | Savage Mark II FV-SR
Mossberg 590A1 20" SP | Mossberg 500 Cruiser