If a head gasket is blown on a specific cylinder then any test for ring problems on that cylinder is irrelevant but it could be relevant on the non-affected cylinders. If a ring problem is present on any cylinder not related to a head gasket breach then you would be looking at either an engine overhaul or a top end job, which means heads and rings.
The latter can be iffy at best because while the pistons can be removed the connecting rods cannot. This means a cylinder hone is not going to go very deep into the cylinder.
(See below)
It is not possible to drop the pan on a Subaru and inspect the piston rings or crankshaft bearings. This is a boxer style engine and doing that would require removal of the engine from the car and the splitting of the engine block. This means a complete diassembly of the engine which of course would mean a complete overhaul in putting it back together.