i have watched some private videos on this that I wish I could share, they do a great job of showing the difference between using engine braking and not.
What is most important is getting the rate of decelleration right. Using downshifts to do so requires a lot of practice and work, but it keeps the engine exactly where you want it in the powerband. The engine braking is not for braking so much as crashaft speed control. Using this as part of your braking strategy is a big plus on the track. The owner of said videos rides about 20mph faster than his group, largely becau he does not have to bang three shifts in a row every corner, instead he spaces them out a bit and this dramatically smooths out his line and his decelleration profile, along with keeping the entire bike spun up. it's multi-tasking to the extreme.
I can't do it wll yet, but I practice it. It's another tool in the box.