Ok, found the bug and some other things... fps override cuts your write down.
The mystery with 10x is semi solved.... the YUV becomes 720x56... in essence it breaks. Is this a bad thing tho? Without fps override in 10x no yuv I got 81 MB/s.
... so its possible to do call("lv_hd", 697); and set fps for 5x, then you can record at 80MB/s... but the image is damaged in zoom mode.. in normal LV its ok to record like this but no point.
So in theory the camera can write to do 1080P+ continuous like 50D.
So all this chasing of crop mode yet the camera seems to record a LARGE chunk of 1728x1152 with maybe some moire in non crop. Normal LV with 24fps.