I opened up my worls in mcedit to remove a bugged block, and i saw that the reactor chasing stores 4 data:
a string (u'BRReactorpart'), and the location of it.

It's really neccesarry to keep them as tile entities? Or at least to store the location once within the block?

TL;DR: Yes, it is necessary. Blocks do not receive many events which are necessary for a fully-operational multiblock structure.

If you're really interested in the messy internals of how multiblocks work, I suggest you carefully read the source code of BeefCore.

Oh, and almost all of the blocks used in Big Reactors are nonticking tile entities, so they have no effect on tick performance.