I'm the poster in question. For the benefit of anyone who hasn't memorized all of my posts over the last six months, I will recap my concerns here.
If anything about Magic needs balancing, it's Summoning (as usual). Spirits are still powerful: with the exception of Earth Spirits, their dodge pools are all better than a maxed-out elf, their attack pools are 10-13, and their Immunity to Normal Weapons in addition to natural armor makes them highly durable. The fact that a magician can have two with a relatively inexpensive summoning focus (Amp Level 2) is more unbalancing than any of the spells I've seen.
Per my post in the House Rules thread, I still think spirits are ridiculously good, so much so that average spirits are basically free Prime Runners, with attack pools and dodge pools that mirror maxed-out PCs. Between attribute points, racial bonuses, and attributes starting off at rating 1, Primer Runners have 27 attribute points. An "average" spirit has 31-33. Great Form spirits would have 41-43.
Gingivitis, I know that you had proposed a drain mechanic a few months ago where the summoning magician had to roll a glitch die each Narration the spirit was sustained to see if the summoner incurred drain or not. Did you ever play with that and, if so, how did it go?
I've had several thoughts about how to balance spirits but since I haven't started playing yet I don't know how well any of them would work in practice.1)
Jack up the opposed rolls to make summoning harder. Lesser spirits resist with 10 dice, normal spirits with 14 dice, greater spirits with 18 dice. (My baseline for this is Coydog, who rolls 15 dice to summon air spirits, and Alyosha Daska, who rolls 14 dice to summon earth spirits.)2)
Make the standard spirit stats the stats for the greater spirits, since standard spirits are already Prime Runners in terms of attributes and dice pools. The new standard spirit will be the old lesser spirit, etc.3)
Riggers can't use their own weapons while commanding their drones (p. 48). Impose the same restriction on summoners as they boss around their spirits.4)
Require summoners to pay shadow amp points for spirits, like riggers have to pay for drones.5)
Some sort of drain mechanic, or limitation on how many spirits can be summoned per scene.
I like spirits and summoning and don't want to lose that flavor. As such, my preference is probably for some combination of #1 and #2, although making the rolls in #1 easier if the rule for #2 is in place. For #3, I'd almost prefer the opposite: removing the restriction from riggers to make drones function more like spirits. I don't have a good resolution for the imbalance inherent in #4 yet.