Mage dps is fine, I don't think it needs boosting or nerfing. It's slow sustained damage, our "burst" comes from pet abilities/clickies/AAs. When the mechanics allow it, mages are pretty solid dps in long raids (if the swarm pets survive and they don't go oom way too early), in the range of beastlords (judging from our Taeshlin parses with Daenar and Sintasia; it's a fight where mages excel). They are less dps when their swarm pets die early, when they can't rain or use pet, or when mobs are resistant to both fire and magic damage. They can't effectively heal their pet so they always have to look out for it (the heal they get heals about 2200 hp with healing inc 8 and 20% tome and specialization bonuses and has 20 sec recast time; the pet with companion health 7+ has 15k hp when buffed, but you'd be surprised how fast it can die - not just because you forgot to recall it on enrage or attack from the back, just from simple whirlwinds or AEs in old content). However that is a minor issue, if they got better pet heals or beefier pets, that would be overpowered. When there are at least 2 targets on which you'd want to rain at same time, make sure you have aggro on both before you cast (either send pet or get close enough for proximity aggro).
No offense, Swift, but this spell doesn't solve the most pressing mage problem: sustained dps on single target when the game mechanics prevent you using rains, which is sadly not that rarely. A spell that takes forever to cast, forever to recast and kills your mana while your spells also cast slower just to see grand nukes like wizards have, seems like a terrible idea.
Mages do have some nukes: relic Bladewind and 64 Storm of steel (both magic based) and 62 Shock of fiery blades (fire). They already are an option, but they are a lot less dps than rains and the fire one is both slow casting and has a recharge time, making it overall a very subpar choice even when fire damage is your only option because mobs resist magic spells.
Here are Tinkaa's numbers over the course of her manabar (1.2 mil charm, Empowered storm 4, dmg inc 8, csi 8, 20% codex and deity bonuses, Critical evocation 2) on Grimstump (negligible resists, everything did its full spell damage, didn't use clickies or AAs for more damage):
- Archaic rain + Relic rain + 2 sec cooldown: 881 dps
- Archaic rain + Relic rain + 60 rain: 922 dps
- Archaic rain + Relic rain + 60 rain + Runic swarm pet: 987 dps (the swarm pet being 190 dps over 306 seconds, I forgot I had others' melee damage turned off so I didn't see the Monster summoning damage; I attribute the lower overall dps to unlucky spell crits and the fact that I lost a couple of seconds casting the three swarm pets)
- runic pet Herbert from front in fire elemental form, 20 dmg weapons that proc icy aura, ancient pet haste, gruplok collar, rbow: 270 dps + Monster summoning IV + III + II from the back: 91 dps - this is all melee dps + pet procs, but it seems low, Herbert is usually high 300s (when attacking from front) to mid 400s dps (when attacking from the back) - Grimstump specific? Herbert's accuracy was 65% due to misses, 58% when taking evades into account; it's usually about 73%, 5% of all his hits were criticals, 43% of all his damage was from procs (innate fire proc, weapon proc icy aura and rbow).
- just chain nuking with Bladewind: 514 dps
Now you'll say that a loss of 300 dps on encounters where you can't rain isn't that bad, but:
- there are surprisingly many encounters where you can't rain because mobs are either mezzed or are moving or you are on aggro list of both, but must only dps one, or you need to be so close to them you'd rain on yourself if you tried to; or they resist fire and magic even once malod/malosinid; or the tank is simply bad and doesn't keep aggro on both mobs
- when mobs resist magic, you're screwed because the 62 Shock of Sun is only half its dps (with casting time 6 sec before CSI, and 2.5 recast time)
- old-school mobs will kill swarm pets (and real pets) with AEs or whirlwind
- the runic swarm pet will cast rains when you're on aggro list of more than one mob (which I actually find pretty cool)
What I'd like to see is how necros feel about their style of damage, since they are also sustained damage with some limitations. Apart from Zake complaining about lack of fast damage in groups I haven't seen necros compain about their raid game.
Another option would be to lower the resist mods on single target nukes and change the DoT spells to an unresistable debuff that increases the damage of your single target nukes cast on that mob by 20% (lvl 44) and 35% (lvl 55) and would work only for the mage who cast it. Then you'd just continue blasting Bladewind on single targets like you already do when you can't rain, it would just be better dps, but still less than rains in optimal conditions, and less than wizard nukes.
I haven't played a pre-runic/tomed magician in about 6(?) years, but I'd like to hear input of those mages, too, if they exist. I remember Plane of Torment being a true torment because the mobs were immune to fire and resisted magic, you can't use fire in deep FR, Tower of Tarhyl, a lot of Shadowdale 6-man mobs are immune to fire and highly resistant to magic (even after malo), outer Prison resists magic a lot, some of Emberflow mobs are immune to fire. I know there are also zones and mobs resistant to wizard and necro spells, but at least they have a broader spectrum of them to choose from.