3840x2160 (16:9) 25 fps 604 MB/sec CODEC?????

Halo all.

I need to render and check material at this size.
ideally actual size and 1/2 scale. I usually use quicktime on the desktop so I have a clear view of system demands/activity.

My lovely Dulce Rx can handle it (the AJA test raid 6 read is720MB/sec #-), 30" monitor can just about show a bit of it. my 7800GT mac G5 quadcore 8 gig ram seems unperturbed.
But- i think- Quicktime stops asking for the data because it isn't happy with custom (huge) 16:9 material so it does a bit and the slows down.
Any one know of an elegant program/codec to let me use this unconventional frame size. I need to render to an uncompressed prefferably 10 bit 4:2:2 or animation type codec to which I can render from FCP.

Would love to be able to do the same size etc. but vertical, also.

any ideas or help with identifying the bottle neck? OR with solving what we think to be the bottle neck (quicktime limitations)?

thanks and happy spring,

Hilary