Setting references to null does not help anything and it is a Bad Pratice(TM), methods variables live in the stack along with the method that defined it. Once the method exits, that section of memory is cleaned and leaves no trace of the reference and thus it is no concern to the garbage collector and those references will not slow by any bit the collection.
About having two piles of methods in the same stack, and having references to diferent objects "confusing" the garbage collector, I would like to see some real data about that as on the stack there can be only one pile of methods, and a method is either pushed over an existing or an existing one is removed and the a new method is pushed in the cleaned space. There is no reference confusion, there is no two methods living in the same level.
About the Garbage collector being slow and calls to explicitly gc being able to optmize that, as was already pointed out, is absurd. Garbage collections are expensive and the more often they happen, the more inefective they are, for that reason the usual strategy is to as much as possible to execute garbage collection and this way execute them as few as it is possible(usually when the memory becomes full). That makes the cost of the garbage collection over cheaper, the flaw on this aproach is that when the collection happens it takes long. That can be bad on systems where you need to be responsive, thus the alternative algorithm is presented, allowing the JVM to collect when the system becomes idle, that does make the system to "seem" more responsive, as there is no freezes sometimes, but this aproach does consume more CPU processing.
In the end, there is no magic, at one extreme you have low cpu consuming algorithm with eventual freezes, at the other and you have cpu intensive algorithm wint no sensible freeze(unless you overloads the machine).
Even if your calls to gc does take efect(which is not guaranteed as per API documentation) you would just push the algorithm towards the processing ineficience with aparent better response.
And then, I would thrust the JVM implementation to do a good job on this, as it is written by experts on the subject, rather than on a programer making assumptions based on personal feelings and myths.