This allows powerful magician characters to still nuke someone in one hit (6 spellcasting + 6 magic + 6 foci = 18 dice, 6 hits average, 5 net hits average) doing Force 6 manabolt, 11P damage (unresisted) which will kill anything short of a dragon in one hit.
This also gets rid of ridiculous overpowered Overcasting. The fact that all it does is turn the damage from Stun to Physical is ludicrous. That is NOT a threat or reason to not overcast. This prevents free insta-kills via Force 12 manabolts. this allows for over-the-top Magicians to still insta-kill via Force 6 manabolts and their usual bajillion damage, resulting in 11P.
This balances the game. Take out overcasting, and be generous with allowing players to buy Spellcasting Foci. This means a Magic 4 mage (slightly above average) with Spellcasting 4 with a Foci of 4 would be able to cast Force 4 manabolts with an average of 3 net hits, thus doing 7P damage.
This is the exact same damage as Overcasting a Magic 4 spell. Force 8 = 8P - 3 resist dice (1 hit) = 7P, but without the extra net hits. This stops people from overcasting 8P + 3 nethits, 11P, suffering only 4DV that is usually heavily (or entirely) resisted.
1) It doesn't change the drain values. In fact, it keeps them lower as no overcasting means no double DV.
2) It bans Overcasting, which is overpowered for such a non-significant negative modifier of changing S damage to P.
3) It keeps all Drain in S form.
4) Allows overpowered mages to still remain overpowered.
5) Allows others more defense against being obliterated in one, unescapable spell.
6) Foci is more important, more emphasized, and this adds to the roleplaying, tactics, and balance of both sides. Also allows the mundane to "knock the staff out of the mage's hand" which lowers their power significantly, adding a heavy emphasis and Lore to the power of Magic and Wizards (the staff helps his power so much!) This allows for those amazing Telekenesis moves on the part of the mage to get his staff back, turn to the Street Samurai jumping at him with a Katana, and obliterate him with a powerful fireball. Epic Story win! Fun for EVERYONE, not just the overpowered mage.
I for one love magic that has a heavy emphasis on magical items. Protective necklaces and foci for mundane to wear as defense. Magical swords for anyone, not just adepts. Magical staffs to channel mage's power significantly. It is more FANTASY!
Overall, I really like this system. It also gives a heavy influence for there to be more mages who rely on cyberware than otherwise. (This is Shadowrun. Half the professional mages should have wired reflexes.) So it not only adds to the Magical High Fantasy aspect of Shadowrun (emphasis on powerful, less rare Foci) but also on the Gritty, Man meets Magic and Machine, type of world (Mages are more likely to have cyberware)