<p>I was wondering if it is possible to tweak the way garbage collector works on JavaMe in order to improve performance somehow(may reducing the number of passages)? I´ve seen some articles about it, but mostly directed Java SE and most of them says GC is highly dependant on the maker. How much would that be.</p>

I don't see any possible way to tweak the garbage collection behavior in J2ME.
If you have problems with pauses induced by garbage collections, you have to do two things:
Reduce the garbage that your program produces, by re-using objects, and avoid costly things like string concatenations (use StringBuffer instead)
Use System.gc() to force garbage collection when a little freeze doesn't matter (for example, during the "loading" screen of a game), to reduce the odds that the collector process is triggered when it's annoying.