I had a derp.
We did some testing last night one vs. one woofer... stock Z v.3 compared to Z v.3 with ONE extra spider (Yellow spider pack). One on one it made a very substantial gain. Have to build a mate for it to try two subs (coming soon)
So we will also be making a set with TWO extra spiders (Red NS v.3 spider pack) compared to stock to see if it keeps going or tapers off.
At lower power the Yellow spiders were about 2.5dB louder than the stock black ones; but at high power only 0.1-0.2 dB louder. So this means we need to look elsewhere to make gains -- such as stronger motors, tweaking the cabin and box, etc.
Very cool information to have on hand, though, in case anyone is doing a lower clamped power class with a similar big 4th order