1. SpellStrike&SpellFire are worth it for an arcane-fire mage.
Yes, that set is one of the best sets you can have before T5/T6
2. Frozenshadoweave set is worth it for shadow priests.
Yes, it is very much worth it for frost mages, shadowpriests, and warlocks (with spellstrike is a deadly combo)
3. Getting the patterns is a problem. (I think some patterns are from dungeons? low drop? high drop?)
The spellstrike patterns drop in instances, I know one is off of murmur in SL, I can't remember where the other one is (I want to say one of the TK instances) The spellfire/mooncloth/shadoweave are all taught by their respective trainers
4. All the above items need having tailoring always while wearing the item? (or just for gems or gems bonus??) How about having the item in the bank and dropping tailoring? (i dont know why I would do that but i´d like to know just in case)
In order to even wear that tailored piece (speaking of spellfire, mooncloth, shadoweave) you must be specced that way to wear it or else you get 0 stats from it.
5. Mage is already almost 63, but my priestess is young, barely 23, i think i will be lucky if i get her to 70 until Christmas. But with the Wrath coming.. I dont know if the FSW set would be wasted?? Did they say smth about new patterns? Would the new mid-end 70s instance blues compare to the FSW set?
I would worry about the mage and just take the time to grind primals while leveling (something I should have done if I would have known I was going to switch to tailoring prior to 70). With the priest just level her as you would but keep an eye on blizzard posts about new patterns, I still bet with a 5-piece set you will probably use it for a good amount of time.
6. Any other stuff I should now about high-end tailoring for oneself?
It takes a LOT of primals to make the sets, so get to farming. Also, depending on the cloth that you specialize in you can make two of that kind and one of hte others (That you are not specced for) all on a 4-day (individual) cooldown.