Your going to feel the first shift (1-2) because of the large change in RPMs. The second shift (2-3) is usually subtle. The third shift (3-4) is usually very hard to detect. As you stated the last one, the torque converter locking, is the most noticeable with a major drop in RPM.
This is for my gasser truck but I seem to remember my dad's powerstroke being similar. His has been bone stock and very well maintained. I usually see a drop of around 200-300 rpm for each shift and then a large drop from the torque converter locking.
It is nearly impossible for, at least me, to know if you have any transmission upgrades without being there. Without hard numbers the words you use to describe it are just opinions. One person may think 3 minutes after a set time is late while another may consider 30 minutes late. The word late describes the situation but can be interpreted differently. I'd look underneath very closely and see if there are any signs of modification. Things like bolts and screws having chips or corners being smoothed from being torqued would be signs that parts have been removed before. They do not guarantee any modification but it will at least tell if they have been touched. Otherwise you'd have to just ask the previous owner or have someone take it apart who would know.