If I adopt this alternative rule (I think I saw this in the "tweaking the rules" section of SR4A)...

... do I must decrease all armor ratings across the board (maybe cut in half) ? It seems logic to me, otherwise armor gets too powerfull. But I dont really know the possible implications of this. Somebody help?

P69 SR4. The option is to do a single opposed test, so no body+armor roll. You roll to attack, he rolls to dodge. If you hit you do DV+hits-(armor-AP).

This is actually mostly very favorable to the defender, as armor+body/3 is typically < armor. For most characters it is a lot better deal. Such a good deal that you are right, it's pretty much over the top. Armor jackets will soak alphas and MMGs completely. And because it's SR, they will soak it anywhere you are hit.

How to change it depends on what effects you want. If you want guys constantly charging into LMG fire you treat armor differently that if you want people afraid of getting shot. I'd suggest building a couple of sets of armor and comparing damage done with the normal rules to see what you want it to do. Do some 6 die pool attackers and some 15 die attackers, as the extra hits make a real difference. Use someone with armored clothing, explorer jump suit and corp FBA and shoot at at them with a pred 4, an alpha and a HMG/panther.

As I suspected.

I have already played with normal armor ratings and it got too difficult to inflict damage. Now I will try halving all armor ratings.

Later I put here my impressions.

Thanks.