You can go as far as .040 inches (1mm oversize) on stock sleeves but anything bigger than that is pushing it. Going anything over 2mm is definitely going to require re-sleeving the block. As for 79mm, I'm not even sure if there's enough physical space for a piston that big in a D-series block. There might be, but the sleeves are gonna cost a bundle.

As far as power gains, you'll be able to feel a 1mm overbore. The bigger you go, the higher your compression ratio goes, unless you put in dished pistons for a boost application.