The 23 hit rating on belt of blasting alone lets you replace 3 +8 hit gems for
+9 or +12 damage gems on rest of gear, which alone is close to negating the spellfire set bonus. (assuming you have around 500 int with spellfire set which would be 35 damage from bonus).
I did the math on my 2 sets, I am arcane spec atm but i deducted the +25% damage from int bonus because that makes the replacement set even better since it has more stats.
Anyway i used robe of hateful echos/studious wraps/belt of blasting and then
spellfire set. Both socketed similar(although replacements had red sockets which are better than spellfire sockets, plus have better set bonuses. These numbers were without any buffs whatsoever.
spellfire set--6633health,9641mana,137hit,27.6base crit,1059 arcane/fire dmg
(this is using spellfire set bonus but subtracting the arcane spec 25% int bonus)
replacement set--7123health,9866mana,164hit,27.8crit,1024 arcane/fire dmg
(once again subtracted the arcane bonus from int)
The big difference is 35 more damage from spellfire, but like i said before if i were to go back to spellfire, i would have to drop gems in other pieces to replace with more hit gems to get back to hit cap. That brings the damage back to about even, and gain stats from replacement.
Take it for what you will, the original poster wanted to know when to replace and here is the hard numbers on it. Not much difference but there are times i have lived in raids with under 500 health and was glad i had the extra stam since didnt have to give anything really to have it.
Thought i should edit this because surely the next person is going to say you dont need 164 hit in arcane spec. I realize this but i usually raid fire (except when killing a'lar and stayed that way for Kael since guildleader wants me strong on aoe for phase 2.) But i used my normal fire raiding gear instead of my arcane accessory gear for this test on comparing the 2 sets. That is why I based results off of 164 hit.
Edited, Mar 8th 2008 2:12pm by Toxicityoh