I'm not sure what a "massive" difference would mean. I think the picture would need to be transformed from pretty awful to outstanding for calibration to make that degree of difference.
The picture out of the box is very good on the VT65 and ZT65, but professional calibration will improve things notably. The best way of expressing it, as stated on other threads, is that professional calibration represents the best upgrade you can make without buying a new TV. (Since you already own what's arguably the best TV available, upgrading your television isn't possible, so that last point is hypothetical.)
Professional calibration won't elimate picture judder, though I'm not sure why you're experiencing this with 24p content. Twenty-four frames per second playback isn't perfectly smooth, though trying to iron this out through frame interpellation (your TV guessing what might appear between frames) isn't the answer.
To the best of my knowledge, your Oppo doesn't introduce undefeatable frame interpellation. Nor does it have motion processing options. You might set the Oppo to output video as "source direct." I think the Oppo has this option. That way your BDP shouldn't add any image processing that might lead to anomalies or artifacts.
In any case, engaging frame interpellation really shouldn't be necessary with 24p content. 50Hz material, especially on Panasonic plasma TVs, is another matter altogether. For UK broadcast material, setting IFC to "min" may help with some content, but you shouldn't set the IFC control any higher than this.
Hope that's some help.