Well first of all the whole Shadow vs Fire question has been answered exhasutively in this thread:
Fire Raiding - The Warlocks Den Forums
- IS it worth beeing fire destro without a fire mage?
In my opinion no. The point of speccing into Fire is to increase your personal DPS - lacking a fire mage is going to negate 15% of that....and of course have you considered the loss to RAID dps by speccing fire?
WoW Warlock DPS Spreadsheet
Has a simulator for exactly the question - I suggest that you take a look at the numbers. If you're trying to progress there's no excuse for not being shadow.
- Does the specc look okay, or have I missed something
You missed a few items but the build Solarus linked is exactly right.
- Is it worth giving up the top position in the damage charts just because Im a bit bored with soulshard farming?
No. Raiding is about repetition a lot of the time, be it shadowbolt spam or SS farming. Its just one of those things you have to get used to. If you're not outputting your maximum possible DPS then the raid as a whole is hurting.
- What about a fire lock as Illidan tank?
Try it. I haven't personally tried it as fire but as long as you don't die and hold aggro then its fine. Are you prepared to switch back and forth from Fire in P1, to Shadow in P2, and Fire again in P3?
- Haste or crit? What stat is the most important for a fire lock?
I'd lean more towards crit than haste, especially in pre SWP content.
At the end of the day I'm sure there are many people would sympathise with the complete boredom generated by the one-button playstyle...but swiching to fire (except perhaps on farm raids) isn't doing your raid group any favours.