I've held off commenting until now, mostly because I have zero experience with mirror-less and 4/3rds systems! As you know, I shoot with a Canon 7D (started with a 30D) in an Aquatica housing - so nothing light or cheap about it, although I realize those are not your primary concerns. With the 100 macro and two DS-160 strobes (single arm section on each side) the rig is about 22lbs topside.
Anyway, I've never owned anything other than an SLR/DSLR so it was a natural extension for me to take one u/w. I was used to the behavior of the AF and most of the limitations of the few lenses I own. Having said that, shooting macro with the 100mm (the non-L version) and the 7D is often as rewarding as it is frustrating. In our local waters, not only are we plagued with challenging vis conditions much of the year, but we also have to contend with significant schlieren given the amount of FW entering the sites at times and the seasonal mixing of very warm water with cold - again, of which I'm sure you're well aware! When conditions permit, and if you're comfortable moving the AF point around (I always start with the center point selected and then change it as required for the comp), the 7D and 100 make a great team. I'm sure the 7DII would be even better!
As you have likely read, the non-L version of the 100mm macro is a pretty slow-focusing lens, but still works well u/w if you can comp with enough contrast - and have a good focus light (in order to assist with the amount of contrast). I've been lucky on occasion to shoot a really small nudibranch in a field of hydroids and had every shot tack-sharp. Had the opposite happen as well... The nice thing is when you nail a shot, you have 18MP (or 20MP if shooting with a 7DII) from which to crop, if necessary. The working distance with the 100 makes it possible to get a frame-filling image without having to get so close that you impact the critter or shot in some way.
The only other thing I can suggest, is that we plan a dive or two together and you try mine. Bring a CF card with you to the site and blaze away getting a feel for the rig before you do anything rash! Oh, it will feel like you're taking a one man submersible with you at first, but you do get used to it. I regularly dive with my rig on tech dives, with doubles, a stage, a deco bottle or two and sometimes a scooter. It gets to be very manageable with a bit of practice!
Hope that helps.