Since there is no electronic ignition in our little 2 cycles, I've always had a belief that the glow plug depth and heat range acted to either advance or retard the ignition timing, just like turning your distributor used to on your old car back in the day. Or if you ever played with a little model external combustion engine, and let the flame get closer and further away, you get the idea.
Compression is the other factor that effects ignition timing, higher compression advances the timing of the boom... so the longer plug not only puts the heat closer to the fuel and compression center, it also takes up a non-insignificant area of the chamber, also raising compression a tad.
In a car, advancing the timing always gave more instant throttle response, retarding the timing killed your low end torque, but gave much better fuel economy, that is why Mallory had the mechanical advance dual point, then vacuum advance came out, then MSD and Electronic ignition, to allow having the best of both worlds with a hot spark and variable timing.
I've always felt you should run the plug that lets you run the fuel you want in order to get the run you want. Oil raises compression too in our motors... but with modern engines, I don't need as much and it kills fuel economy. I don't want to HAVE to run 15% or 20% nitro in my engines, so I run enough compression and mostly synth oil to work with the range I like, 7% to 10% most normal conditions, and 5% for King Orange weather. I can pretty much run 7% year round by switching between an Enya #3 & Enya #4 in cool and hot weather, only needing 10% like at Muncie in July heat or similar days in Florida.
I did some experimenting a while back for a friend and I made a simple tool for measuring glow plug depth, because he was getting flame-outs, and we found we could minimize his particular issue with a particular plug that reached deep into the chamber and protected the element (and Arden hose nose Hot I think? I forget the model number)
But after all that, for me personally, it boiled down to me pretty much running the box stock setup that Randy has recommended from the beginning of time... 5% to 10% nitro range, mostly synth with a bit of castor fuel, Enya #3 or #4, depending on the weather, set the pipe by the chart, pitch the prop until it works, then forget about ever touching it again and go fly.
EricV