Can I just tell my tuner how to make my boost curve?
They should determine it, not you, based on the compressor map of the turbo you intend to use
Or does the n75 determine that?
No. The brand of thermostat you use does not determine the temperature of your house.
Got a new uroparts n75 and it boosts good up top. But lousy down low. The old oem one boosts good down low but lousy up top!!
I can try a new oem but they r $$$$.
If the tune will end up overwriting those n75 changes then I probably have nothing to worry about??
There is nothing in either of those two paragraphs that makes any sense.
Get a functioning n75, one that has not been fiddled with by a windowlicking moron or sold as an "upgrade" by a fucking scammer.
Then the ECU needs to be tuned properly for that N75 to make the boost that the ECU requests.
There are two different functions in the ECU
1) determining target boost (this is has nothing to do with what N75 you are using)
2) controlling the n75 to achieve that goal (this depends on the target boost and the kind of n75 you are using)