again, as many of the above posters i am no expert....

however this is what i have gained from my pylon racing experience.

Stinger diameter worksbest when smaller than inlet size ( the whole back pressure story), but only if there is first sufficient volume as in a tuned pipe system. As well as tuning the length of the header and pipe combo, you can also 'tune' both the stinger length and diameter. There are so many theories the best way is just to experiment and see the results first hand, as that is what works for you with xxx motor and xxx setup etc. the variables are so great it may be different for the next guy. dont forget change the prop, heat range of plug etc etc and all the setup starts again.

if you are running cannisters or a tuned pipe make sure the header pipe is of equal cross section to the exhaust outlet.

try adding a length of say 70- 100mmlarge diameter silicone tubingto the stinger on your std mufflers, I have done so and found a reduction in noise, but also a marked drop in rpm in some cases. I never checked head temps because it never resulted in any gains so i just removed it and never flew like that.

I think if you are referring to reducing the outlet of std side dump mufflers with small volumes to increase pressure it could be erring on disaster. I havent tried it so maybe not, but i would watch temperatures very closely.