I bought the Bosch kit for $28 off eBay and put it on today. Very easy to install with supplied wiring diagram and no wires need to be cut, thay either fit together with spades to the solenoid and ignition wire or bolted on the starter for voltage source or block for ground.
I had a vintage Hella can relay with mounting bracket lying around so I used it instead of the supplied relay, this requires relocating the pins in the socket to comform to the old style layout but it's easy to do.
Works flawlessly. So does it make a difference? It sounds livelier when cranking but if this is wishful thinking on my part at least I don't have that amperage going through the old ignition switch wires anymore.
I mounted the relay to the PS bracket: