[Jonny Reload]

On pg. 182 of SR4A it states that Force limits how many hits you can use in your dice pool upon the Spell... But right below that it states that this limitation doesn't apply to Edge dice used to boost the pool.

Can you cast a Force 6 Spell and get a few extra hits on your Edge dice to give you 8 or 9 hits but sustain it with a Force 6 Sustaining Foci? (Ex. Increase Agility 8 or 9 being sustained with a Force 6 Sustaining Foci?)

Also, what if you decide to roll your Edge before you roll all your dice, are all your 6's that you re-roll count past whatever the Force of the spell is?

[Juxtamon]

...I think so...that is, Edge results can go above and beyond the Force max for successes, and they don't actually increase the Force of the spell, so a Focus could hold it, but you'd have to be grotesquely lucky.  Certainly not something to depend upon.

   It'd still be dispelled vs. Force 6, though the hits needed to skak it out would be higher, due to the gross number of successes in the casting, with Edge spent there.

[Bradd]

I think you could reasonably rule either way on this. Normally I'm inclined to be generous to players, but I also hate bookkeeping. If you want a justification to disallow it, you could rule that foci can only sustain Force, not Edge.