Racing Thermostat 154 Degree

when is necessary use and change for lower Degree Thermostat(68c) in subaru ej257

for exemplo in sti with 300Whp i can change only radiator for a big and keep the original thermostat open with (78c), has some problem, or if use lower Degree thesmostat together with big radiador I has more chances that my engine work in lower temperature ?

Are you actually racing the car or just driving it on the street 95% of the time and taking it to the track once or twice a year?

More or less this, the car has 3" exhaust system, cool air intake and remap, little increase power, in track day the temperature is little high and I changed the radiator for big mishimoto, well, i use the car 100% on the street, and 1 time per month I go to track day, so, my question is, if i use the orignal thermostat will has problem during the track and in during the use on street or is necessarily the use racing thermostat when use big radiator ?

A racing thermostat isn't going to make your radiator or oil cooler cool the engine any better. It just lets you run at a lower operating temperature if your cooling system can support it.

Exactly, the thermostat value is where it starts opening and they are normally fully open 10C above that. If the coolant temperature is above the fully open position then it is the cooling ability of the radiator that will regulate the coolant temperature as the engine load (heat put into the coolant) varies.

A quick check on that radiator suggests it should be good for quite a lot more power than you are using?

What do you mean by "is a little high"? It is normal for the coolant to increase a bit under track conditions and, unless it gets close to the red, I wouldn't worry about it too much. Are you using anything in the coolant or just water and a corrosion inhibitor - anti-freeze and other products will increase the boiling point of the coolant significantly, if you are worried about boiling the engine?

Oh, what sort of fan mountings are you using - I have seen several very poorly designed ones on YT, by people that should have known better, where a shroud that blocks large areas of the matrix were used.