So I got the chance to look at both long and short uppers closely.
It appears that barrel behind gas block is 0.85 to 0.87" ish in dia. They are essentially the standard C8 barrels that they have been making. The 15.7 is essentially a UK C8 SFW barrel stuck in an IUR. It retains the same barrel sleeve which gives it the thick appearance, including the inverted crush washer.
The gas block is the same as the MRR - it is not like the Danish gas block that was pinned down. They are now fitted with 2 set screws at the bottom, even though the IUR still retains two holes that appear to be for gas block pinning.
My guess is that the IUR will end up the same weight as C8 SFW with RAS. It doesn't have the reinforced gas block for mounting HK GL, but the hand guard is much longer.