@Fenix Wolfheart
I think you misunderstood my noticing magic roll attitude, I was talking about a lot of things so it may not have been clear. I do allow someone to notice a spell being cast its just that you'll be noticing the hand geastures, verbal comments, effect of the spell. If your facing away from them you don't get one then to sense a feeling of dread. That is if you can see the mage casting you get a chance to notice the mage casting even if you aren't looking but it wont be something like glowing eyes, a pulse of energy. There's enough subtle magic (invisibility, illusions, sense X) where I feel allowing a role just because its magic would hit magic with an even bigger nerf bat than just going from 4th to 5th has and render a lot of spells useless. Yes magic can have an effect on the physical world but the simple mana = physical effec is usually far beyond anything a mage is likely to be throwing around and their actually channeling said mana into an effect already. Making someone invisible, creating a fireball, crafting an illusion. That is the effect on the physical world not having their eyes glow or the like.
With regards to magic rules yes that is a houserule. Normally someone can only see a spell being cast as above when its in effect or if its cast in a way they wouldn't see e.g. you duck round a corner so your out of sight and cast insivibility then they will not get a check. The two exceptions are other mages and people who take the "magic sensative quality" (currently priced at 13 karma). The quality functions the same as the sensing metamagic with the addition your allowed a reactive roll to notice magic being cast or targeted on you. That is if someone you can't see e.g. your facing in a different direction when a mage casts invisibility the GM makes a hidden observe in detail perception test and if you succeed they say you sense x (depending on how you want your magic sense to work e.g. pricklin hairs on the neck). Same if someone casts clairvoyance targeted on you you'll sense it even if you don't know what it is. Obviously if you come into range of a sustained spell you don't get this check
Now this is different to the powers description (the magciain can't use this ability to sense astral forms, spells, mana barriers, spirits or foci). However I don't feel its to big of a change to adjust that line to "The magciian can't use this ability to sense astral forms, spells that are being sustained, mana barriers, spirits or foci". Essentially it gives someone the ability to sense (notice) if a spell is being cast along with the other major natural effects (background count, aspect, etc) because your a mage who's trained their magical senses (initiation + sensing metamagic) or your naturally sensative to mana (Magic Sensative quality). All the normal every day person on the street can see someone waving their hands and chanting or a physical effect like that person suddenly becoming invisible.
I priced magic sensative at 13 because that's the minimum value for initiation so for a mage its only worth taking the quality if they want to spend their free metamagics on something else and it wont get them the max magic increases or other benefits from an actual initition.
@Overbyte
I have noticed a weird tendency for people to nerf/hate magic for its effects but not bat an eye if a technology method gives the same bonus. "Your magic fireball hits everything in X area for Y damge that's overpowered. Yeah my fusion grenades do the same thing so what?"