Spell penetration at 80?

Are spell penetration enchants/gems necessary at level 80 in pvp? I'm having a great deal of trouble getting full burst damage at 78 in PVP on some classes/specs- what should be a good 4.5k+ lava burst crit is more often than not being reduced to under 2000 on many classes - priests, mages, and DK's in particular, but sometimes others. I know there are spell resist talents, but looking through the talent trees I am finding nothing that seems to reduce magic damage by a good 75% consistantely without shields. Yet I constantly have to lava burst + em cl + earth shock + lightning bolt 3 times to get a 78 or 79 dk down. Granted they are packing a good 18-20k hp at that level, but the math still doesn't make sense. I'm seeing it on high level priests and mages as well.

Has there been a significant change in spell resist mechanics, and do I need to change my gear accordingly? Is there any change at 80?
Or do I just have bad luck and these -3% spell hit mechanics are making 2-3 of my shaman miss?

Thanks

When I upgrade my Cloak I will be placing Spell Pen on it. When I feel my resil/stamina is at a sufficient point, I'll place some gems adding more Spell Pen. It is quite a valuable assest to our arsenal but nothing that takes priority.
The 35 that a cloak offers pretty much negates a Mark of the Wild, the problem is Arcane Mages with a base resist of 140. Or a hunter that throws up Aspect of the Wild. It would be nice to have 100+ spell pen, but other stats at this point in time are too valuable.