Without that, I suspect they will be less reluctant to invest in a 10- million dollar non-pivotal study.
How do you know 306 is not potentially registrational?
Pain is a valid clinical target for an NDA submission. It may be harder to hit, and impossible to get an SPA for, but that does not mean it can not be a basis for an NDA.
The commercial value of this might on it's own be questionable, but as you say they already could be on the market so the broader indication could be of more value than a stand alone pain NDA.