In all the motors I know of that have been tested, I've never seen in-frame Snipers make less power than the other pipes.
In fact, my new 4mm motor made more/better power with the Sniper in-frames than Shearer out-frames.
That said, LED has played with some various size stingers as well. And I know Cam has also tested some.
My in-frame LEDs have a stuffer that goes in between the expansion and stinger that did make a slight difference in RPM, but we didn't see any difference in temp.
There's a whole lot more info about this shit but NGAF really so I'm not going to bother wasting my time.