For a perfect non-leaky pressure relief valve, the action is like an on-off switch. As pressure is lower than rated, the air path is 100% blocked.

Operating under the rated boost, there should be no difference whether the hose is clamped or not if the waste gate is non-leaky. The fact that the boost can be built up faster indicates the waste gate is a leaky device. Weak spring mentioned by psfred is one form of "leaky" device.

__________________
Shane83SD