you can see the difference, Subaru tstats are notably beefier than aftermarkets...folks have posted pictures online comparing them side by side, huge difference. find a picture and if looks like the big beefy one instead of the 4 year old toy variety then you're probably good.
i don't use them but it's not like they all fail, plenty of cheap aftermarket tstats work fine too, so i wouldn't obsess about it.
make sure you got all the air out of the system, these engines notoriously get air bubbles that stop coolant flow and cause overheating.
all that being said - this motor has headgasket issues, which you already know. unfortunately you bought Subaru's worst engine ever, a little research on any Subaru forum would have showed the extent of the headgasket issues of that motor. and you bought one that was previously overheated (HG's were already replaced)....that is a very risky purchase, there's no telling if the block was compromised at all.