Boring out pre cups?

[6.9power]

Okay, I was reading on here and somebody was talking about boring out the pre cups as another way of lowering the compression without milling the pistons. Did somebody on here say that they had tried it before? I would like peoples opinions on this, as I haven't heard a lot of talk about it. I like it just for the fact that I don't have to yank the engine and tear it all apart to do it. I know a machinist who specializes in head work, so he would be able to do it. Any clue how much they would have to be bored to get compression to about 21:1? Any major advantages/disadvantages of this vs. milling the pistons? I would like to do this somewhere around September probably.