I have no idea what pressure an IBC could take so can't help there.
Actually, it's more than you'd expect. IIRC, they're tested for their UN certification up to 1 barg. However, this is the pressure they're designed to take without bursting - you'd find it will turn spherical before then, which isn't ideal!
If you want to try it, you could pressurise it carefully, but not to the point it starts to distort, and that would definitely help your flow.
That said, if you have the wherewithall to pressurise up IBCs etc, surely you also have the capability of installing a small pump, which seems like the better and safer option to me.