First I'll address your question about higher SPLs in my Medley-Pacing set. I don't think of 15-16SPL in BK or 8-9 in BR as high -- though I'm certainly capable of lower.
Rather there are two ways to think about the potential energy cost of lower SPLs.
1) Fewer strokes means lower Stroke Rate, which ordinarily will mean lower Heart Rate as well. . . However . . .
2) At some point a lower Stroke Rate allows a bit too much inertia to creep into the stroke . . . which means the need to generate more power to overcome it.
So the central goal of any set or practice where you purposefully vary stroke count is to strike an artful balance between a leisurely/restful tempo and conservation of momentum. Because you do this by feel, lots of practice -- and frequent changes in stroke count -- are best for honing your feel.
Your version of the set of 8 x 200 Free that I'd done Monday
is an example. In my set from Monday I kept the same SPL range throughout, but added one stroke to the 2nd 100 of each to help with even pacing. I made the 200s gradually faster in part by swimming fewer of them between recovery breaks of 4x50BK.
In your practice, you sensed that you could achieve a better pacing pattern next time you try this. That's information from feel.
Here's another challenge. The differential between my 1st (straight) and 8th (broken at 50s) 200 was only 15 seconds. The difference between your 1st and 8th was 41 seconds.
Descending sets offer two levels of Master
Improve your time steadily within the set
While doing that, improve overall average by closing the gap between fastest and slowest
So now you have a goal for improving the Pace Mastery Circuit
this set is designed to hone.