In addition to your biggest damage sources from the 2p, you also get a flat 10% increase to
Fire Blast,
Phoenix Flames, and
Fireball which means that you basically just have 10% more damage. In addition to this,
Fireball also crits 10% more, which is nice. This is like a permanent stack of
Pyrotechnics. The crit chance for
Fire Blast and
Phoenix Flames is not really relevant here.
Frost Mage Tier Bonuses
These set bonuses will generally not have any impact on a Frost Mage's rotation because
Ice Lance casts against frozen targets and
Frozen Orb are already high priority spells in the rotation. The only talent build that may see rotational changes is one that specifically tries to avoids ever casting
Ice Lance by relying on talents like
Deep Shatter,
Wintertide,
Slick Ice, and
Cold Front to massively increase the strength of
Frostbolt casts; however, being encouraged to actually spend
Fingers of Frost with this build thanks to the 4-piece bonus is actually a positive rotational change. A minor concern for all builds is that the 4-piece bonus could encourage waiting to spend
Fingers of Frost (increasing the chance of overlapping it with
Winter's Chill) to improve uptime of
Touch of Ice, but with its current tuning I do not expect that to be an issue.
Early simulations show that the 2-piece bonus can be expected to increase damage by between 2% and 4% and the 4-piece bonus can be expected to increase damage by between 4% and 6%. The specific numbers are slightly affected by the number of targets, but are mainly a function of how well selected talents synergize with the set bonuses. Talent builds utilizing
Freezing Winds will typically reach the higher end of those ranges (around 10% damage in total), while those that select
Glacial Spike will be on the lower end (around 6% damage in total).
Overall, this set bonus already seems to be in a good place in terms of tuning and is mostly meeting Blizzard's stated goal of not significantly altering talent choices or rotational decisions. Unfortunately, there is one area where the bonuses fail to achieve this goal, which is due the lack of synergy with
Glacial Spike: casting
Glacial Spike means spending time that does not interact with either bonus at all. If
Glacial Spike were a strong talent this would be less of an issue, but it is already looking to be a generally weak capstone and this set bonus causes it to fall even further behind other choices. The simplest way to resolve this would be adding an additional effect that has synergy with
Glacial Spike.