I am planning on buying the high end 27" with 64GB of ram from OWC.
But I'm not sure if the I7 is worth it or the I5 - will the I5 give me a massive performance boost?
Buy the i7 and the best graphics you can afford; neither can be easily upgraded down the line. CG/VFX will be CPU & GPU heavy so those will be your main bottlenecks. Of course get the lowest possible amount of RAM as you can upgrade that yourself.