best to do it with heads off engine. get them all the same indexing positions that way with no doubt. then # the plug for install later.

i noticed a smoother idle and running engine. it will help performance. how much? results will vary.

over kill on a daily driver IMHO, on an all out high HP engine, they still do it today on the bigboys.

yes,you may need a few set to get 8 out of them.

would i do another engine? sure. if time and budget allows for it. i like tinkering with stuff like that on the bench to keep busy when building engines. in the car with headers on? i would not bother.