In know there are many that see this as an upgrade but in fact you are just adding another failure point. For all of our vans the original design has lasted for 23 years + with maybe a few switch failures. Things fail. 
The really does nothing to help a slow turning starter. All it does is help get the starter solenoid to pull the gear out and make the contact for the starter motor. The other point of failure is these relays can fail closed. That makes for bad day. The relay used for this should be of a type designed for an inductive load (arching) and be of a double break, spring loaded design. If installed in the engine compartment or otherwise outside it should also be sealed or at least enclosed to keep most weather out. 

Slow turning starters are usually a sign of a bad battery, cables, or the starter itself. For an upgrade ask for 87 and later as they are slightly higher horsepower, (.8 instead .7). I know Bosch is our preferred brand but I have had many over the years that were not right. The relationship of volts/amps/power gets very complex with these types of motors but if a new starter is slow to turn testing needs to be done to establish why. I find that if I get a bad starter it is usually a batch problem so the next 3 or 4 will also be bad. I have recently had very good luck with starters and alternators from NAPA and AutoZone. 

Merry Christmas
Dennis