I have the Grizzly SX3 as well, and I use the CNCFusion Deluxe kit with optimally 0 backlash. I use a KelingInc 640 oz-in Nema 34 on my Z, and it is OK with my motor settings of 80 IPM and acceleration of 5. I am running it somewhat derated at 3 Amps Bipolar Series, so I am not likely getting much more than 500 oz-in from it. However, it will only work this way with the assist of the gas spring - without it, the motor would frequently stall.
My initial thoughts are that using the stock leadscrews might need somewhat more torque. Although the mechanical advantage of the acme leadscrew is higher than a 5mm ball screw, the friction is undoubtedly much greater; especially on the Z with the heavy head (even with the gas spring assist). I would try loosening the taper gib on the head, back down your IPM and accelerations settings until you get reliable movement; then tighten the gib back up to normal. If you then stall out with the same motor settings, I would think 400 oz-in is just underpowered in this application. Perhaps backing speed/acc down more will help.