Hi Henrik! As always, great post!
I've done some work with DT_INTS, and in ASM mode, they are fairly fast, as they only copy a few registers, vs the PBP type, which copies then restores about 70 registers with each interrupt. So I don't understand your warning about going above 4 Mhz? I've run it at 40 Mhz on a different chip and it worked pretty well using ASM type DT_INTS.