Pushing coolant out the overflow tube

I'm having an issue with coolant pushing out the overflow tube on the coolant tank on my LS7 when I'm on the boost. I can drive it around town normal without getting into the boost and it never looses coolant. Nothing in the oil and I don't see or smell anything in the exhaust. I've done a leak down test and all cylinders look good with no bubbles in the coolant. I did the leak down test with the motor warm and then again with it cold to see if there was a difference but it still tested ok. I've replaced upper and lower coolant hoses plus added a spring to the lower in case it was collapsing under boost. Any ideas on what else it could be? Motor temp is always good and the system will still have pressure in it after the car has sat for more than 24 hours. Running 10 lbs of boost and the motor is all forged with ARP head studs. I know under boost its seeing way more than 100 psi like the leak down test uses so anything else I should check before pulling the heads to install new gaskets and check for warpage? Would a bad thermostat cause something like this? Short bursts of throttle don't seem to be an issue its more when I stay in for more than 4-5 seconds. It may be the heads are lifting after a while under boost but I'm hoping to avoid pulling the heads if it could be something else. Thanks for any ideas to check.

Pushing coolant out the overflow tube