Hi!
Using as much nitro as 15% (which is totally unnecessary,5% is all it takes) and a tuned pipe demands a good tank! That means getting a Tettra "Bubbleless tank", or if you don't want to invest that amount of money...an ordinary tank set up with two clunks (Called Uni-flow set-up)
It's also important to understand that the more nitro you use (or the higher the compression is) the colder glow plug you have to use .
My experience is that Rossi 6 or 7 plug...or even the coldest there is an 8 might work .The Rossi 8 was more or less the plug to use when I flew F3D pylon 12-15 years ago. (we used 80/20 fuel, no nitro).
You will notice that the colder the plug you try to use the more senitive the engine will become.
Adding a head shim (0.1mm) will give the same results as using lower percentages of nitro!