What’s your position on issue versus personally procured equipment?

Two major points of concern here: Mission effectiveness; that is, is the issued gear up to the task? and unit cohesion, readiness and discipline.
With regard to the first point, if you are deploying with experienced troops who know their gear and are well versed in operations - who cares if they all wear the same t-shirts or if they have a gerber tucked into their boot? On the other hand, if your unit has a significant number of young soldiers, uniformity of gear will make equipment much easier to track and manage, and standards will be easier to maintain. If everybody is doing the same thing with the same gear it actually helps the newbies cope better with the frenetic environment.
If you are not deploying everybody ought to be on the same sheet regardless. All the A-bags should be packed the same way. Same goes for the B-bags, etc. The training environment needs to be tightly controlled and that means no rogues. You do NOT want surprises in the field during an ARTEP. It's the soldier's privilege to be surprised, not yours. You need to know that everybody in your unit has the proper gear and that it is serviceable so that training will be on an even playing field. Every unnecessary variable is an additional potential point of failure.

I think it depends on the reasons and the mission. I know that when downrange, with a small-ish group of mostly NCOs, SNCOs and officers, all of us had "personal" equipment. Some guys were coming from units and rates where they had their own issue...and others adapted their IBAs, IFAKs, LBE, etc. based on personal preference as time went on. We were in DCUs, so nothing "matched" the ACU bits anyway...so that wasn't much of a concern for us. The "Geard-do" thing was the butt of many jokes...and largely avoided (or abandoned after the first razzing). There's also some great aftermarket stuff that's lighter, warmer, "better"...common sense becomes a virtue if it makes things run smoother. I would assume that for larger units, with more junior personnel, particularly in a training or garrison environment...common sense has to be supplanted by uniformity, just to get everyone at the same minimum standard. The NCOs and SNCOs rightly "own" this topic...but my humble opinion is that in the field, if it works more efficiently, and doesn't compromise readiness, or the intended protection of "uniform" gear...I'd say good to go.

Ditto- we all most always modified our gear for the field- Metal snaps replaced by 550 cord,pistol belts closed with same to get lower to the ground.