Ok, looking at the wording on Rapid Growth and the General Timing call out here's what I got...|
If the model that has the Rapid Growth upgrade is the acting model that kills Papa Loco AND is within 2" of the nephilum to allow for growth, the growth takes effect. Otherwise Papa Loco will always blow up before Rapid Growth would take effect.
Explanation:
Rapid Growth only gives the abilities listed to the model with the upgrade, the abilities in turn effect other Nephilim, but they do not *have* the upgrade.
So the acting model kills Papa Loco, and following the timing rules the acting model's abilities are resolved first during the "Killed a model" event. So if the acting model is the one with Rapid Growth then the Feed the Brood effect kicks off first. If the acting model is anyone else in the Neverborn crew only their "When killing a model" abilities would activate. This is followed by Papa Loco which is, of course, blowing up. If one of the Nephilim kills him, they have to wait until the third step in the timing occurs, other models friendly to the acting model. So if the Nephilim survives the explosion they could still grow as normal. But they could be killed before growing.
Any rules savants think I got this wrong?