How much sealing do you need? Subarus are known for blowing head gaskets. Let’s accept it and move on.
So, you upgraded to ARP2000, but if you want to play on Ethanol with the big boys (400+ whp, track use, etc), you will probably want to spend the extra $450 to get the 625 studs. I learned this empirically tracking my GD and GR. I had fresh rebuilds on both of them, and they were both pushing coolant out the overflow. These cars were not crazy (430WHP, 550WHP), but I would take them out for 10 laps at a time in SoCal summer weather. Rarely saw temperature issues, but the heads would lift and push coolant out the overflow tank. We went to SEMA (2014?) and asked all the motor experts (sleeved motors, head gasket manufacturers, etc). Common response was “you need to retorque the heads,” which is a tall order for a Subaru. We tried it on both cars anyway. We did get a quarter turn out of the ARP2000, but cars still pushed coolant at the track. No signs of deformation on the headgaskets, just coolant leaking past.
This kept me off the track for a while and ultimately led me away from sleeved motors, which both cars had. I upgraded one car to 1/2″ head studs and the other to ARP 625 head studs. Based on 2 years of track abuse, I have no reason to think the 1/2″ (more expensive) upgrade is necessary. My EJ22 putting out 550WHP exclusively used on track with ARP 625 has run two seasons now without refresh (though I’m likely jinxing myself for this year).
IAG’s website hits on one very important issue. If you’re going to start playing with fasteners, you run the risk of distorting your mains. Increasing the clamping load creates all kinds of weird distortions in the block; so, spend the extra money to line hone the main journals of the case halves while bolted together with upgraded case studs. Maybe with the headstuds/heads too? I’m not sure.
Lastly, O-Rings create new issues as they tend to leak with common MLS head gaskets. I have yet to hear of a solid track build using o-rings that didn’t have a leaky gasket. Maybe spraying copper all over the head gasket would work? That copper O-Ring needs something to bite into. I simply don’t have enough empirical data to opine on the use. I do still believe sleeved motors can be used if built correctly from the ground up, but, again, extra machining is not optional. Put all your fasteners on and machine. I tried to build a sleeved motor that had seen better days last year. Machined a bunch of stuff, but didn’t do it in the proper order. The cylinders would go three thou out of round when everything was clamped together. Sadly, pistons are not egg shaped.
Just some thoughts. I’m probably 20% accurate at best, but what do I know? I was raised with girls.