I think you are confused with sizes. Based on the ACT specifications for that flywheel the kit included 8mm bolts for the cover. In fact ALL the ACT flywheel for 2017 WRX have 8mm mount holes for the clutch cover.
The flywheel to crankshaft bolts are 10mm and about 50 ft.lbs. sounds about right for a single torque - older ST/WRX were about 54/55 IIRC. I think the Subaru spec for that model is actually a torque angle for the OEM bolts.
You try to torque those cover bolts to 50 ft. lbs. and you're going to be pulling the transmission or engine back out.