Berserking

Unless my math is terribad...

Berserking = 20% Haste for 10 secs every 180 secs = 1.111111111111% Passive Haste = 36.43 Haste Rating

Didn't see this here anywhere.

While my trollish Mage with 348 haste rating (10.61% haste) using Berserking and Icy Veins (also 20% haste) together my Frostbolts only get reduced down to 1.44 second casts and not the full 50% I should receive.

You are correct that using portal is probably the best way to prove the effectiveness of berserking on spell casting, but I think you are misinterpreting how haste is supposed to work for spells. 50% haste means you get 50% more casts within the time period, not that you cast spells 50% faster. Indeed, 1.44 is the proper cast time for a frost mage's frostbolt.

Here is the formula: Final Cast Time = Base Cast Time / [(1 + Item Haste)*(1 + Berserking Haste)*(1 + Icy Veins Haste)] = 2.3/[ (1.1061)*(1.2)*(1.2) ] = 2.3/1.593 = 1.44.

This hastens the global cooldown.