need to see pics. I'd put them back in the car and leave them. Unlikely the pieces are (primarily) cosmetic.
The front most piece/cover helps keep water from getting on/or inside the bumper cover and the hood latch hardware.
I went downstairs and took a look at my Boxster's front trunk and the plastic panels/covers and found in the holes -- one each side/end of the front cover -- some trash that might have if the cover had not been there gone on into or behind the front bumper cover. Over time this could result in a large amount of trash (mainly plant debris) behind or inside the bumper cover. Damp or even wet trash/debris. Yuck.
Now my Boxster doesn't have those side pieces that run along each side of the front trunk and the only pic of my 08 Cayman S I have of that area the pieces are missing cause they were removed when the car was torn down for evaluation after an accident. The 03 Turbo's front trunk looks like my 02 Boxster's, that is it has the same front most cover/panel and the same cowl covers but the areas on either side of the front trunk are body metal (painted).
I note too the front most cover fits very snugly so much so that the water running down the side of the trunk can only flow on top of the plastic cover. It also seals the gap of each headlight module. If it didn't, was missing any rain/wash water would run down and overflow into the the gap between the headlight and the body/fender/bumper.
This suggests very strongly to me the desire is to keep water (and the debris the water can carry) out of the area protected by the front cover.
The side rails may help keep water away from the seam/hardware where the fender bolts to the shelf of the body that runs along side the front trunk to prevent corrosion.
The pieces are there for a reason. I'd put them back and put up with the tiny amount of bother they cause when you wash/dry the car.
Sincerely,
MarcW.