I know from my "sphincter dyno" that flat changing in the RX-2 when it was turbo'd definitely gave a gain in acceleration over the traditional lift-shift-re-accelerate technique.
There's a definite knack to doing it without killing the box too, but once you get it right it's great. In the RX-2, just before you shift, you put very light pressure on the gear stick, and you find as you approach your shift point, you press the clutch, and as soon as the clutch begins to loose pressure you feel the gear stick go light and you begin to pull it through. By the time you get it to the neutral position the clutch is almost disengaged, and it will pop into the next gear without any problem at all.
You will get a slight flare in RPM, but when you get it all coordinated, depending on just how much power the engine has you should be able to keep it down to a couple of hundred RPM give or take. With the rev limiter set at 7,600rpm I could shift at around 7,200-7,300rpm and not touch the limiter.
There's no need to grab the gear stick like a gorilla either, I used to use two fingers wrapped around the gear knob and that was all the pressure needed to shift. I did use the longest gear stick I could find as well as this makes the shift action very light. Forget about the argument about a short shifter only needed to move the stick a shorter distance, because the effort and notchiness involved removes any advantage. I'll promise you can shift a light load twice as far just as fast as a heavy load half the distance. Look at the V8 Supercars, even before the sequential box came in, and see how long a gear stick they use. Short shifters are for boy racers, real men have long sticks
There is also a lot to be gained from getting your clutch set up with the right amount of travel, as often you will get disengagement in say 70% of pedal travel. So there is nothing to be gained in pushing it to the floor, you are simply wasting time and effort. When it's all set up you just dip the clutch rather than pushing it all the way in and again it works a treat.