IIRC Blizzard has disabled/redone the nat pagle algorithm, so some of the changes won't work.

1) I think I saved about 100ms when I did the registry tweak last time, tunnel service like gamepath reduced it by another 100, bringing comfortably to 300-350. I recommend doing both if you haven't.

2) Sometimes the spikes can be graphical, especially when you are melee class on the boss so you get front row seat view of all the spell effects. Also adjusting your view distance settings for bosses like felmyst can help.

3) Also, like remi said, clear your WTF and mods occasionally. A lot of raid UIs really lag or even disconnect your PC, especially in SWP.

4) A good IP can help with the spikes. I alternate between a low latency but spiky IP (65.49.x.x) and consistent but slower IP (65.50.x.x or 218.111.x.x). Test out your connections, some fast downloading IPs are bad for WoW, and vice versa.

5) Finally, my friend had been spiking despite lowerping and registry tweaks, and his PC is amazing (Crysis and WiC at max detail). He complained to TMnut until they switched his port, and problem solved.