I've just added :SetTPSSamples(samples) - until I add an interface for it, you can use:
/run AceLibrary("Threat-1.0"):SetTPSSamples(30)
Or whatever value you prefer.
I've been thinking about adding a fight-long TPS number, but I didn't really think that would be valuable to anyone, since you typically need TPS numbers so be able to evaluate relative threat velocity.
Edit: It was easy to do, so I just added a second return value to GetTPS(), which returns TPS over the entire encounter.