I also raid 6 hours a week and started 3 weeks late in Firelands, my group is 2/7 heroic and should get a few more this week, 20% nerf is way to much for heroic, normal its fine, but they should leave heroics pretty much the same, if they are to hard, nerf each one individually so its kill able by 15% of the population or whatever they want.
My theory is Diablo 3 beta is coming out that week, and they want everyone to clear firelands in 2 hours then go and play diablo.