Do you think it's worth it in terms of what it does to gameplay (ie during multiball I think??)

Speaking of TZ, I turned a standard production TZ into a prototype version by drilling a hole through the playfield and adding a working 3rd magnet. 

A few others have done it now with the prototype playfields being remade, but only a handful had been "pinsane" enough to go through with it as a home made proto version.

Rare, maybe not. Unique, for sure.