You wrote:
" ... Under these conditions my Mugen 1800mAh battery would last about four days, consistently. The MPJ one under the same conditions? A bit over five and a half. I couldn't believe it. ... "
Ι can't believe it either. The MPJ rated for 2500mAh is 40% larger and should have lasted for about 6.25 days but you only got 5.5 where is the missing 18 (extra) hours? Incidentally, the battery performance you're getting is phenomenal to start with, I am happy if I get through one full day with similar use like you. I have invested on wireless charging (base and cover, about $35 total) so when at home or at work I just place the phone on its charging cradle and keep it topped up.
Microsoft and Nokia better come up with a solution to this, it is unacceptable for a modern smart phone to not last one full work day, even with moderately heavy usage.