As much as I like having the first ADR, I always seem to get better pics when we have slightly later pre-park opening ADRs. By 8am, most people are trying to check in to their restaurant, tour, or BBB...so, there's often a temporary lull in the pre-park opening crowds around that point which can mean people free pics by the Partners statue and the castle. Of course, you will be finishing up your meal a tad later.
I would get to MK by about 7:30a to get in the line to be let in for pre-park opening ADRs, tours, etc. We've always been let in between 7:40-7:45. We usually finish up right about 9am or just after when we've had the 8am slot, but really not too much later with the slightly later slots. Since they've started letting regular guests in around 8:45am, you won't really have issue with getting around in the park after your ADR. Considering the popularity of these ADRs, I personally wouldn't book it just to ditch it. I mean, it's nice to be in the parks early and see things with very low crowds, but aside from the common courtesy aspect...I don't think it's so cool that it's worth being assessed a $10 pp no show fee to just keep walking around and skip the meal.