Ok , just for you who were interested , those are my conclusions :
1) meth has much lower caloric value then 98ron , therefore you need to inject about 40% more.
2) this can not be done with FSI injectors since we are limited by injector capacity.
3) if running very high quantities of METH through WMI or additional injectors , the FSI injectors will pull back since the Lamda sensor will sense much higher fuel concentrates , this means you will then need to run more meth through aux injectors.
so overall , it is possible , but a real headace. add to that that the fuel system needs some parts change (meth + plastic = damage) and an additional injectors management will be needed , i am backing off right now.