I would not be overly concerned with pressure buildup inside the spoiler. It isn't going to be enough to cause it problems. The extra holes are probably there for manufacturing concerns.
If you are worried about it, you can run a bead around most of the spoiler and leave a few selective gaps, and don't seal the extra holes. I would think low spots where any water that does get in can get back out. Perhaps leave the points on the bottom open, while sealing the upper arcs? Of course, you would have to leave a long enough area that the silicone won't bridge the gap. That would allow for expansion as well as the exit of water, without giving water an easy path in, even when the trunk is open.
I've had mine on since late August. Not a lot of time to see potential problems, but I really don't expect anything.
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Steve
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