Ah.....that will look great.I opted to remove the Lumma spoiler - and not to go with a spoiler at all.
One thing you "might" want to consider changing, is your rubber antenna with a shark-style like I did. It will look exceptional, most especially on your black stripe. I bought mine a year ago, brand new off eBay for $19 and free shipping. I found my eBay sale page, but the seller must have run out of stock.
No modification is required to your Sky in any manner. There are 4 Shark connectors (3 on the Sky) of which 3 color-match the Sky harness. Cut the 4th unused connector with wire off the Shark. For the remaining 3, cut off their plastic shrouds and plug each one into the appropriate color-coded Sky connection, then wrap with electrical tape to prevent accidental disconnect and also to isolate the shrouded ground casings from the steel deck lid in case there is any technical issue with intermittent contact. There is a cast metal base piece on the Shark that is easily removed so that you don't have to modify the rear deck lid hole in any manner. Easy peasy, no mods to the Sky itself, and is easily reverted back to the original antenna if you later change your mind. I tested it afterwards and the OnStar and XM radio work fine with the Shark.
It is surely a matter of taste, but I feel the Shark is nicely proportioned to the size of the rear deck lid. Not only for style, but also for durability.
If you have an OEM spoiler, the Shark will NOT fit because of an interference.