Hi,
As you say you were fine on "rocket" fuel, change the fuel and you have problems, in an attempt to overcome problem you have increased the MJ size , do not forget that this only accounts for flat out throttle position, as mentioned by another post you most probebly have a problem with over advance/mixture/ Octane problem but much further down the throttle range, many peaople forget this. I suspect the damage has been done whilst pulling between 1/4 1nd 1/2 throttle where yor may have too much advance to support your octain rating/mixture and thus the piston is already glowing with pre ignition when you start your flat out run causing piston failure.
Always set up your jeting/ignition map to suit the LOWEST octain fuel you are going to use , this way you will only be loosing a very small less than 1-2% of potential power when you go onto high octain fuel BUT IF you can not get this then the engine is safe to use.
When racing MX we used different maps/jetting to suit different octain rating when ever we changed fuel.
Make sure that you have the correct card to siut the octain rating of the fuel you are using as these control the ignition advance map to suit the fuel octain. Change these cards around to suit fuel in the tank. Alternativly you can use a detanation sensor placed into the cylinder ( much machining involves) and use this to control the retad of the ignition timing when it senses detonation. This is an expensive route to go, maybe you should just keep running high octain fuels.

BR.
Dave