this is a quick tab of the benefits of the spellsurge enchant...
some facts:
-100 mana over 10 secs to each person in the party of the effective enchant.
-can proc once every 30 seconds.(cooldown of 30 seconds)
-3% to proc, still procs pretty much every 30 seconds.
so, once you have about 1600 healing you're beginning to get into the overkill zone, i personally think that between 1500 and 1600 healing and you should be fine, then you should start looking into other stat gems depending on class, spirit and mp5 as a priest, mp5 and spell crit and a paladin, spirit and mp5 as a druid and god knows what as a shaman, i dont really know what they work off of.
i have 1700 plus healing which is overkill i would much rather have a bit more spell crit to get over 25% holy spell crit chance but my gear only has 4 gem sockets 1 of which is a meta socket which my meta gem(the halfcast gem) requires more blues than yellows. so as i get different gear ill change it as well as i can.
but for the calculations, lets say that spell surge procs once every 30 seconds.
thats 200 mana every minute.
so 200 mana/60 seconds*5 seconds(each tick)=16.666 reapeating MP5. now this doesnt seem like much for a single person, but when you get into the 25 man raids it gets bigger.
3 tanks
8 healers
15 dps
7 casters
7 mellee(hunters included)
this is just an example raid.
so you have 15 people that this could help.
you of course try to have a tree druid and a warlock in the group with your tanks so they essentially get 33.333 MP5
you have 4 of your healers and if you're smart a shadow priest in one group for the mana battery. which gives those 5 people 83.333 MP5. then ideally another group that is exactly the same.
then you have 5 more casting dps which they also get 83.333 MP5 for the each person in the group.
Now i know that this is a perfect situation, the enchant procs every chance it gets and you have the perfect raid build, etc etc... but, i am clearly convinced that if you have between 1600 and 1700 healing you should sacrifice the 81 healing on your mace for spellsurge.
i dont know how many people have tried this but a guild called Wait for Saunderz on shattered halls has required that on atleast one pve epic healing weapon that you get out of a raid you will have spellsurge. i dont know how well its working out for them seeing as how i havent talked to them in ages but im also convinced that spellsurge is actually better than 81 healing assuming you have enough plus healing already.