Take a look at the glow plug, see if the element is starting to come out of the glow plug bore. If so this is an indication that the engine is detonating. A "misfire" is not usually an indication of detonation, well not unless it is preceded with over heating.

Now most sport engines do not have a great deal of blow down (time in degrees that the exhaust port is open yet the intake port is closed) to allow the tuned pipe to supercharge the engine. So unless your engine has a high compression head to begin with or you are using rather high nitro, the pipe's supercharging of the cylinder is not much of an issue with detonation. This is assuming the engine is propped for the same rpm of higher as it was with a muffler. Loading the engine down into a lower rpm with the pipe set up to resonate at lower rpm then detonation can become a concern even with "Sport Timed" engines. As you are running in the 14k to 15K rpm band you look to still be in the normal rpm band for the engine. I seem to recall that the OS46AX is rated at 16K rpm

Most European engines have rather high compression ratios as they are designed to run on no or low nitro fuels. So adding a pipe even with sport timing can cause detonation issues. But your OS is a sport engine designed to run on moderate nitro 15% to 25% max fuel I'd look to adjust other tuning factors before shimming the head.

I like to use as cold a glow plug as practical. This forces you to lean the fuel mixture to get the best on pipe high rpm. This often results in a lean midrange. A lean midrange is often needed as the higher fuel flow needed at full power from a piped engine will flood the midrange, or off pipe performance.

I'm glad to see you are trying to use a high performance set up ( I think all pipes are beyond the scope of what is called a sport engine). But I have to make clear that a pipe does add a lot of dynamic variables that make tuning them rather tedious. Again the piped engine is not for the beginner or for those that aren't willing to put in the time to understand what is happening.