There are 5 problems with the approach you tried.
First, if you have Elemental Mastery, you're an elemental shaman, and your Lava Burst base cast time is 1.5s, not 2.0s.
Second, haste effects don't reduce your cast time by the haste you have. Rather, they allow you to cast more spells (or more melee swings) in the same amount of time. For example, 10% haste doesn't mean your 2.0s Lightning Bolt cast time is reduced by 10% -- it means instead of being able to cast 100 Lightning Bolts in 200 seconds, you can cast 110 of them. To figure out what this makes your cast time,
2.0 * 100 / 110 = 1.818(not 1.800)
Third, haste effects are multiplicative, not additive. So the haste effects you list should be
((1.05 * 1.20 * 1.01 * 1.30 * 1.2113) - 1) * 100 = 100.40%(not 5 + 20 + 1 + 30 + 21.13 = 77.13%)
Fourth, you shouldn't worry about gearing for temporary haste buffs like Elemental Mastery, Berserking, or Bloodlust / Heroism.
And finally, I think you're not really interested in what your cast time is with your current gear. I'm pretty sure what you meant to ask is, how much haste can you get before you reach the 1.0s global cooldown cap for lava burst. For that, you want
((1.5 / 1.0) / (1.05 * 1.01) - 1) * 100 * 128.05716 = 5307 haste rating (or 5488 for non-goblin)
which is high enough you shouldn't worry about it.