Thanks for answer. By any chance, do you have any idea how to find some of the discussions you mentioned?
I'm sure they're buried in the history of the forums. It can be tough to sort through, but the search function does work for the most part.
But the conversation basically boils down to two primary interpretations:
Power Foci add to your Magic rating or they add to just what is on that list.
They are very powerful foci that temporarily increase your effective Magic rating. That means they add to your Sorcery, Conjuring, and Enchanting dice pools, along with any other test where Magic is involved.
It's all based on those two sentences. The increased Magic side of the fence argues that "increase your effective Magic rating" means exactly that, while active your Magic rating is increased. The second sentence is just a reaffirmation of what having a higher Magic rating means.
The more limited effects argument is based on the concept that an increase to your effective Magic rating isn't explicitly defined. That second sentence, therefore, acts as the definition. The first sentence is not the rule of how the focus works.
It hasn't been further clarified officially, so it is primarily up to individual GMs, but the more commonly accepted interpretation is the more conservative one. That ruling being that a Power Focus only adds to your Dice Pools and nothing more.
People in the first camp are often hold-overs from the older editions (third and earlier, I believe) when Power Foci explicitly stated that they increase the user's Magic Rating. In fact, I just looked it up in a Second ed. book I have access to and it very specifically says that it increased the Force that the Magician could cast at without taking Physical Drain. However, the wording has changed dramatically (even if it is now less direct in terms of what it does) and magic works very differently both in-setting and in-rules now than it did in earlier editions. Both of these factors are straight-forward reasons to expect that the way Power Foci works to have changed.
There are two problems that I see with Power Foci as they currently stand.
1) (assuming that they
don't add directly to your Magic Rating) Power Foci don't do anything unique, they just combine several other foci into one. This is a two-edged sword, on the one hand, with enough money and karma, there isn't much reason to have many of the other foci if you have a Power Focus. Specifically, Spellcasting, Spirit, and Enchanting Foci all just add their Rating to a Dice Pool that includes Magic, therefore they wouldn't stack with a Power Focus. At the same time, it also means that someone that has already invested in several of those foci has no need for a Power Focus, thus devaluing its effectiveness (and therefore its hold on the title of "very powerful foci").
2) (assuming they
do add directly to your Magic Rating) Power Foci can be extremely overwhelming, the ability to summon higher force spirits and cast higher force spells without risk of Physical Drain. These are not things to take lightly, they might not seem big at first glance, but you might be surprised at how much it can start to matter quickly. Plus the ability to stack up multiple foci and get huge bonuses on a dice pool is nothing to laugh at.