Rapax M5 Advice

[Masterosouffle]

Question for those with a Rapax (or Ares) if I can, what spring are you using for what joule? 
Currently I am running an M130, and achieving 1.8j - which I believe is about right - originally I was losing a lot of air through a bad seal, which is hopefully rectified - the concern being that a poor air seal would require a more powerful spring and therefore more stress on the gun. 
The concern I am having is I keep being warned or hearing of issues with the gearbox casing cracking, and that I should run at a lower power in order to prolong the life of the gun. Obviously I would rather not do that as it's an awful lot of money to make such a compromise, and the site I play at is huge and very often windy so the extra power is not something I would give up lightly, however, if I am sitting on a time bomb then I will have to drop down.
Thoughts and experiences if you don't mind.
Cheers

[Masterosouffle]

Just to add, I had been experiencing misfeeds, and noticed the lips of the maple leaf bucking which had been fitted recently were protruding into the feed chamber, I cut them back (badly), and the fees issue seems to be resolved, however I now have an increase in power - the opposite of what I was expecting, with 350fps over 2j with the M130. Have ordered a new bucking to cut down cleaner, but I can't understand where the increase in power has come from