your 25 man mob would be the #1 target on any server you roll on, and would get chased down by a ton of people. mb'ing in WoW is not as easy as it is in EQ (they have removed a lot of the stuff that allows bottish type actions) and just 3-5 well played characters could easily take down a mob of 25 controlled by 1 person. For example, if you have 25 mages just a few hunters on you with high arcane resist pets, your pbAOE wouldn't hit them, pets would be heavily immune (esp bestial wrath) and chew you up, and with a lot of strafing they'd be out of forward line of sight making you very frustrated when trying to do direct damage spells (just positioning 5 correctly is hard, 25 would be insane).

try 5 man first and grow from there, 25 man doesn't really buy you much since most of the raiding is heavily scripted and requires very precise gameplay thats hard to multibox heavily, and in PVP you will get destroyed a lot by much smaller teams.