If your current trunk lid has no spoiler and you replace it with a trunk lid that has a spoiler, you'll need to replace the dampeners (struts/shocks) that hold up the trunk lid. The spoiler adds enough weight to the trunk lid that the dampeners need more capacity to hold up the lid.
If you get a trunk lid that has had the spoiler added, it may have been located in a non-factory location on the trunk lid. If it was placed more to the front, it shouldn't be a problem, but if it was placed more to the rear, the trunk lid will be heavier than the dampeners can support.
From my experience: I have a '98 SVT and it did not have a spoiler. I bought a factory style spoiler (with the brake light) and then eye-balled it for placement on the trunk lid. I put it where I thought it looked best. Turns out it's more toward the rear than the factory placement. SO--even though I replaced the dampeners with the factory dampeners for a spoiler application, they don't hold up the lid. I placed the spoiler MAYBE 1 inch farther back than factory. That's enough to throw the weight off and keep the stock dampeners for working properly.
Good luck!