All you have to do is hold down brake then tap the gas on a down shift.
I wouldn't know how efficent it is as I've not tried it with a mouse, but I'd assume it is no different do actually doing heal toe with a set of peddles.
Well, when I'm lazy enough to not to setup my wheel on the table I play with mouse. I blip the throttle when downshifting and cut it while upshifting I even use manual clutch. Of course it is efficient. When done correctly the car is much smoother and more predictable on the brakes.
its not heel and toe its just a throttle blip, since you're not actually heel and toeing. and yes it works and its certianly effeciant. partyboyu used to do it all the time with his mouse skillz
When I don't have my wheel around I play LFS with the mouse and I can easily blip. It will be easier to get the hang of it if you press your clutch button down for a bit longer than you usually do. Just takes a quick click to do it right.
The same goes for keyboard also. When I had no wheel I had gas brake and clutch on the keyboard and steering on the mouse. Blipping works that way too.