(I don't know if this is any new info to you guys but I found it quite interesting.)
I found out that JPEG buffer is almost unlimited* when shooting stills in video mode. Raw buffer depth is the same.
I know there is a crop(either 4:3 or 16:9) and file size is slightly smaller(around 5-10%) but it still doesn't explain to me why I can only get 3 JPEG in a row(without slowdown) in photo mode. Even with the lowest quality JPEGs! Maybe there is some logical reason and I'm just being dumb. Don't be harsh on me. xD
I don't know if this is any useful info for the development but I thought I would share this.
E: *I tested this a bit more. It is not quite unlimited as I first though but it's still much better than in photo mode. Buffer depth seems best in 4:3 video mode. I can get around 75 shots(shooting large jpegs) in a row without slowdown after formatting and reboot. After that I can get roughly 25 shot bursts. In 16:9 mode it seems to be slightly worse weirdly(bigger crop). I only have 40 MB/s card though, maybe it is better with faster card.
E2: Shooting medium or small size jpegs buffer seems almost unlimited. Got over 150 shots and I stopped myself. Didn't want to ruin my shutter any more.