I have used all four techniques (hi-tap, unwinding wire, PBR jacked up coils and bridge-rectifier) on various games. In my limited experience:
1) Hi-tap made the least (but still noticeable) difference but is the easiest to do or undo and gives a little more ooomph to kickers, flippers, etc at the same time.
2) Unwinding 3 rows made a more noticeable difference and as stashyboy said, if you are careful cutting the paper, no can notice.
3) PBR jacked up coils were similar to unwinding (with less work but more cost) and also made a clear difference.
4) Adding DC bridge rectifiers (which I would get from Radio Shack before the closed them all by me) made the most difference such that you could actually feel it through you fingertips on the game! (not a shock but just a more solid fee). Only downside is it takes much more soldering, I had to buy the rectifiers and it is a bit of a spaghetti nest under the hood. However, I did it on the five pop bumper on my '64 Williams Heat Wave and it really flies!
All of this is after rebuilding the pops (or flippers), cleaning everything etc, not a shortcut. I'm kind of with EMsInKC, I like a faster game and all of these are reversible if you want. It is important to factor in the playfield layout. For example, on my Sing Along, I felt like the pop bumpers are so close to the side rollovers that I was worried it would radically change the play of the game so I did nothing but Hi-tap the game.