5x ele shamans are VERY viable in TBC, it's what got me started boxing after watching a vid of them back in the day wrecking AV lol.

As for being viable in classic, I really can't say how they would do in end game dungeons. I remember when I was leveling mine back during TBC I was able to plow through leveling dungeons no problem. Bosses just melted, but once I hit lvl 58 off I went to Outland to blow more stuff up with the new TBC greens that were as good/better than Naxx epics lol. Never got a chance to run Vanilla end game dungeons with Vanilla gear.

That said by chance I just started a 5x shaman team today, pretty fun so far, got to level 9 before the wife pulled me away from my game lol. Not sure if I'm going to stay elemental with all 5 or try out a 5x enhancement now that IWT was kept for Classic.