Great post DC DOG!
A few thoughts. Ive extended 4 m4's with the short tube by adding Freedom Fighter or Daves 7rd extension tubes. I had to go back and install Wolf springs to all of them because sooner or later they experienced failure to feed issues because the stock 5rd spring was to week. While adjusting the elevator down a bit seemed to be a temporary fix, installing the Wolf spring was the only real "cure".
Adding anything aftermarket requires fine tuning and vigorous safety checks. (especially in the trigger mechanism)
Moving the magazine spring retainer to its furthest outward position is a given when installing any spring, this insures the spring has full usage of the mag tube length. Aftermarket followers are another thing to keep in mind when trying to utilize the full potential of your tube. Oversize followers take away from your inner mag capacity. Many springs are tapered on the end because they were designed to fit inside the follower and not just rest behind it. This eliminates any sharp spring end from scraping the inner wall of your mag tube. Keep in mind when trimming a spring/fine tuning it, be sure to bend the cut inward with a pair of needle nose pliers. This goes for both ends of the spring, tapered or not.
When trying to fit 7 rnds into your new 7rnd tube, the differences in length per ammunition brands do vary. Dont let this train of thought limit your rnd cnt in your tube! I made that mistake. I found out that after installing the Wolf spring that 6 rnds of anything was the most I could squeeze into my tube, and I shot it that way for along time assuming it was just the brand or length of ammo. After realizing that the 7th rnd always was just about 5/8ths of an inch from going in the tube, a simple removal of 2 1/2 coils was the ultimate fix for me.