Here my $0.02...
Take the spoiler off and use the little push pin plugs (you can get them at auto parts stores, I got mine at Lowe's) and a little bit of silicone to seal off the holes. Everything you need will cost you about $5.

If you find you really like it, but the plugs bother you, get a painted trunk lid or have the holes filled and painted. However, if you're like me and are not 100% decided on what spoiler you eventually want, just keep the plugs in and wait till you decide.

I knew I didn't want the factory spoiler - but I also didn't want to wait 8 weeks to pick up my car. So I took the factory spoiler off and I love the way it looks. Now, if I go with a spoiler like the Roush or the Cervinis, they will use the factory holes. If not, I can have 'em filled.

 

I can see the bolt heads from here and I am not likin it IMHO
But then again, its not my Stang.
The more suggestions the merrier...

What ya got, two different size tires on that pony?
The sidewalls look different in heights from front to back.
What sizes are you running if ya dont mind me a askin?

 

I also looked for ages for someone with a trunk without the spoiler to do a swap. In the end, had spoiler removed and body shop fill in the holes and repaint.

 

Looks like he's running some beastly drag radials in my opinion. Looks sick! I thought my slicks looked intimidating, but those are the best looking radials I've seen yet.