Running a ej20t, standard computer, new genuine subaru thermostat, factory fan control, heater circuit and bypass is all there.
Curious to know why the temp initially shoots up to 115 then drops back down to a happy 90?
Anything to do with the o2 sensor being 2 inches from the turbo and 2 inches from the cataytic converter?

maybe it takes that long for the thermostat to open

It's nothing to do with your oxygen sensors

What is your heater setup?

If you have a really big heatercore or a long pipe run to it you will get that problem

the stock thermostat uses the return from the heater to control the 'stat.
So if its really big and sheds too much heat or the pipe is long it keeps the 'stat shut that bit extra till temps equalise

Hmmm my temp dont shoot up at all???
does vary from about 90 to 120 .
thing is with cars is they dont over heat.

The heater circuit is connected, but without a heater core- just the output connected to the return, but the hose is about 600mm long (hard to see, but it's the orange one in the first pic)
Noticed when i put the thermostat in that it had no small hole in, which I've seen in other cars.
If it's not normal for the temp to initially flare up like that then I'll keep looking for the cause

The hole in the thermostat is only for air bleeding when its first filled.
They actually supposed to have rattle pins in them so the hole blocks once the flow starts.

check to see that you have all your water pipes running the same way that they would in a subaru, air bleed hose and the turbo hose to the header tank etc.

I had this problem too which is why I'm going with an external thermostat. As said, it's because you have such a large reservoir of cold water on the return/rad side of the theremostat which keeps it cool and therefore opening later. Once the system had warmed up a bit I didn't have the problem. There's a Canadian kit which spaces back the return and bypass flow which apparently can work too..