Since it requires only 41 Megabyte/s write speed 1536x656 at 24 frames/seconds should be possible on the 650D. At least for more than a few seconds.
"Should be possible" is a funny thing when it comes to software/hardware performance. 41 MB/s is the average write speed over some amount of time. There is no accounting for variance. If the write speed slows down for a few frames, you don't keep up with the buffer and drop a frame. Also, that statistic, I believe, is gathered using a different method of writing to the SD card than RAW video. No matter what math you do, it can only do what it does.
There may be some optimizations available somewhere, somehow, but the code looks pretty straight forward. Copy to buffer, write buffer to disk. Getting other stuff out of the way (off of CPU) may be the best option for improving performance.