Hi
300+ is as you noticed to high for an X2 and volume spacers only increase ramp up not initial sag
You can thou put spacers in and run a bit more sag with more ramp up to prevent the shock bottoming out
I have found X2s do like a bit more pressure than most shocks.
Unfortunately the X2 runs quite high pressures. Did you cycle the shock every 25 psi as you filled it to equalize the negative chamber?
A Float X might be a better option. You'd be at about 220-225 psi (max is 350).