Potentially kind of a munchy question and one I couldn't find a good answer too with my weak search fu, but do Power Foci add their rating to Arcana and Enchanting tests? I can't find something in street magic where it says no.
By BBB pg 192: A power focus adds its Force to all tests in which the magician’s Magic is included.
By Street Magic 83: Researching a focus formula is an Arcana + Magic (Force x Force, 1 day) Extended Test.
By Street Magic 84: Crafting a focus is an Enchanting + Magic (16 + Object Resistance, 1 day) Extended Test.
The reason I consider it a little munchy is that it leads to the cycle of building better foci using previous foci. Of course, it's a waste of karma, but with a power focus 2 and middling enchanting/decent arcana at the start, it's not _too_ hard to get a rating 4-6 power focus designed and enchanted, especially if using the optional rule that Air spirits can add Force to foci design rolls (36 successes extended isn't that hard with 13-16 dice).
After that, pay the exorbitant karma cost to bond the power focus 6 and you are suddenly a lot better not only at making other foci but at flinging spells and summoning spirits. Oh yeah, and you can do all of this in about two or three weeks with almost no real cost unless you get a few glitches on your design and are required to use exotic telesma.
Anyways, for my own table, I'm not sure if I'd allow it or not, but if anyone knows of some obscure (or really, really obvious and missed by me) reference in Street Magic for why power foci wouldn't add to those rolls, I'd appreciate the info. Thanks in any case.