Yup, the conical requires that maximum or almost maximum loads be used, there is, after all, a lot more surface engagement on the conical than there is with the 'belt' around the ball.

As for shooting it with a nitro load, I'm not sure about that, as I was always of the mind that the nitro conversion was designed to shoot ball, rather than conicals. You might like to talk to Mr Westlake about that, if he is the one who converted your ROA.