I have completed the weapons holster and am onto the last upgrade for the syringe holder and a couple of others. Some of them I have only done one upgrade on so far such as the bait bag and the heavy ammo, but I don't tend to use them a lot so not my priority at the moment! I wanted to be able to hold 4 weapons and as many syringes as possible quickly so that I could have all of the tools available when attacking the outposts.
I don't race through it, I really enjoy taking it all in too

as I say I just play a lot of these kind of games so I do a lot of the collecting part whereas a lot of people just do side missions and main missions and wouldn't bother with things like collecting the masks, collecting the letters etc. You tend to find with a Ubisoft sandbox game that about 50 to 60% of the progression is the story, then the rest is all of the collectibles and side missions... Which is the part I really like!