From my point of view of wanting a whistle that will give my bronchitic lungs a reasonable workout, what I’m learning - correct me if I’m wrong - is that there are two distinctly different ways in which a whistle can do that for me. On the one hand, it can provide a decent level of resistance requiring my lungs to push against it. On the other hand, it can require a decent amount of air and require my lungs to take nice deep breaths and work constantly to keep the air flowing. It will be interesting to discover what balance of these hits the sweet spot for my lungs - in a low D whistle with smaller-hand friendly ergonomics. Haven’t heard anything so far that would suggest the MK is a bad choice in that regard.
The whistle I returned (and that is now returning to me due to the customs mix-up) is a Chieftain Thunderbird with 3 bodies - D, Eb and E. Hole 5 on the low D was just too big for me. However the Eb was playable, if difficult (big stretch on the lower hand, bigger than on the D), as was the E. I returned them thinking that it wasn’t a great investment as it was the D that I would need for the majority of tunes. I’m beginning to wonder, though, if my budget can stretch to it, if I might get the MK for my D, and hold onto the Thunderbird and actively find tunes for the Eb and E. I’m not sure how much was resistance, and how much was air requirement, but my lungs certainly felt well used after playing any of the Thunderbirds for a while. And I do love it’s big, hoarse-throated sound - almost like a shakuhachi at times!
On a completely different note, I’m coming from a fiddling background (though intermittent over the years and never well developed), and I have a fair bit of traditional music that starts on the G string. I’m thinking of getting a low G whistle to play it - looking at the Chieftain V4, but would appreciate other ideas in the same budgetary range. Ideally a decent volume “chiffy” whistle that plays well in the upper register, even the third octave, to get as many as possible of those 1st position E string notes on the fiddle (up to 3rd octave B on a G whistle).