I don't remember anything about a 2mm rule, but I do remember there's language in the rule stating you should rack "as tightly as possible". If you know you can get rid of that 2mm gap, then you should, regardless of whether you think it will help or hurt the breaker.
To answer your question, there's no truly wired ball in 8b the way there is in 9b.
There's a ball that the top pros might make 70% of the time, in both 8b and 10b, and this percentage goes up or down depending on the equipment. That ball is the 2nd row ball into the side pocket. It works on a barbox too, and you can alter the path a bit by moving more towards the center or the rail.
The catch is, you need a truly perfect rack and/or new equipment to make that ball anywhere near 70%... think magic rack + diamond smart table. If you're dealing with a beat valley bar box and plastic triangle, forget about it.
On those tables, the best bet has always been a second row break, a little over a ball width from the rail, and trying to avoid an airborne cue ball as petey notes. I don't hear it discussed much, but the ball that goes in all the time with this break is the opposite corner ball. You see this in slow motion when someone 'gets lucky' and sinks a ball on a 1-pocket break.
What I think happens here is...you're hitting the very edge of the head ball a millisecond before the rest, and this nudges the corner ball down towards the foot rail. Then the 2nd row of balls chain reacts and sends a ball from the back of the rack into the just-dislodged corner ball, and that kisses the corner ball into the pocket. Something also seems to bank into the other corner (on your side of the table) too.
What you do with the cue ball is up to you, you can draw with low outside but with a heavy cue ball you might get unpredictable results depending on whether you touch the head ball on the way in or not. You can also use just a 'touch of draw' or even low inside, with the goal of sending the cue ball two rails and back towards the middle of the rack. This is one way to get the 8 ball on the break.