The force ERT teams across Canada uses the IUR with the M203A1 sleeve...
It may not be given to all members but some are able to customize their rifles the way they want (I agree that a majority will just use what is given to them). All that is needed is the boss approval/signature, witch is given after a 5-10 mins chat. This is not to spend more on useless stuff, it's to suit the rifle to the operator so it can be used to it's max potential. I'm not talking patrol rifle that will be used once a year to run a qualification course.
In regards to the "90 rounds carried" you never know when & how many rounds you'll have to use to neutralize the threat(s). Different scenarios means different tactics & obviously # of mags carried, you may have to use more than what's carried in the pouches anyway (a partner may have to feed you some more...etc).
I've fired both M4 & C8. I prefer to have the more accurate tool to be able to engage at longer distances (if needed) than "hoping" to make my shot with a hot, used & abused less accurate tool... That's just me, you would probably be fine with your 6920 as with a majority of members. But a C8 is way more accurate than an M4 past 50m weather it fires it's first or 200th shot in SA or FA, that alone could make the difference between a neutralized threat vs. a critical situation. While at it, the profile of the basic IUR w/o the CF M203A1 sleeve is no heavier than the M4. You're saying that it's a full heavy profile, it isn't, it's a medium profile (about 0.76" behind the GB and it thins out to about 0.65" up front) witch makes about the same weight minus 0.4". What's heavier on the C8 is the railed forend. Skilled vs. Skilled operator, the IUR makes a better allround rifle. That's just my opinion, no hard feelings.