Just keep in mind that the gain pot has two functions in this circuit:
Lug1-Lug2: Increasing resistance in the negative feedback loop of the first op amp increases gain (this is how most op amp dirt circuits have the gain knob)
Lug2-Lug3: Increasing resistance before the 2nd op amp REDUCES gain.
So, using a higher value pot increases both resistances if the pot is half way up, so the effect will be offset. However, it should still accomplish what you want, because when the gain pot is maxed, Lug2-Lug3 will be zero ohms, so the gain will be higher when using a higher pot. So, my point is using 250k will allow for the possibility of higher gain, but throughout the sweep it might act differently than you expect.