Just closing this thread, since the correct answer is to implement the 6's in the wound roll and do 2 different operations and the summing them up in the end to get the complete answer.
Now if anyone wanted to know how it went then here is the general rundown for this strategem:
Sine this is a probabilities game:p then the following is true. Making the attacked model do a 5+ save....
6/4 of the time the saving model will fail. So most of the time^^ But having only a 1/6 or 6+ save then allmost all of hes rolls will fail.... that's why weapons that has a high or semi high ap that gets even higher on theyre ap value isn't so good, since they won't change the probability for getting a wound through by much.... The probability of the roll failing is so high anyway from the start.
But taking a weapon that has 0 ap and going to town with those kinds of strat is a big deal.... again becouse of how probability works....
For 50 shots with a bolt rifle should give around 1 - 2 more wounds by useing the strat.
But using it on a normal bolter ie ap-1 in tactical and making those 6's change the weapon to ap-2 should net you about 4 more wounds in general by useing the strat while shooting 50 shots... that's a 50%increase in extra damage from the strategem but with the same amount of shots as with the bolt rifle....
Rerolls and other modifiers to hit or wound doesn't really change the strat in many ways.
It ofc gives an increase in damage potential and therefore isn't a bad strategem, but the gain of it is rather small, and if your low on CP then I wouldn't use it even if its only 1 CP, it does increase in Worth if someone has the ability to shoot again. The intercessor strategem for instance.
Allso the potential of the strategem depends a lot with who your firing on ofc, and what your firing with:
Against MeQ in cover so a 2+save its actually worth its points since its 1CP for some extra dmg. And your changing the sv roll from a 4+ to a 5+ with a bolt rifle. Or even better from a 3+ to a 4+sv with a normal bolter.
It doesn't make you go through hoops by useing it either, and it's armywide aswell and last through the turn, not just a phase. And your going to fire those weapons anyway^^
The strat allso becomes better the more RF or assault weapons your firing: atleast 50+ shots to make it count.
Lastly I would say this isn't something your going to use every game and probably not many games either, but if the situation is there and you have enough models on the table still to benefit from it, then its not the worst thing to do either. Every extra wound done is extra wounds there still firing back in your opponents turn;)