chase

Short answer, nobody knows for sure.

Counterspell is a binary spell.

On binary spells hit/miss/resistance are all rolled into the first roll.
Looks something like:
spell hit * resist % = X
Server Roll < X = Hit
Server Roll >= X = Resist/Miss

Now it is never mentioned in the blue post if the hit cap is taken into account in the first roll on binary spells.
If you can exceed the 99% hit cap in the first roll than exceeding the hit cap increases your chance to land the spell.
If you are capped at the 99% hit cap in the first roll than any +to hit beyond (3% for pvp) is worthless and only spell pen (and talents to the same affect) will help you break resistances.

There have been several people who have claimed that exceeding the +hit cap for binary spells (notably frost) does indeed decrease the resist rate. (noted for bosses, and binary spells breaking the boss resist) However, every time I have looked into the claim the data behind it was nothing but smoke.

So you can either belong to three camps:
1) Spell hit beyond the cap increases your chance to land binary spells. So arcane focus is great for landing arcane binary spells. (sheep, CS)
2) Spell hit beyond the cap does nothing for your chance to land binary spells. If you want to break (non-boss) resistance your only hope is spell pen and lots of it.
3) Little faires determine hit/miss/resistance or we just don't know enough yet.


I'm in camp 2 and 3. The only reason I grab arcane focus is because it is the best of the worst talents to grab on the way to clearcasting.