Yes, it does a great job of cleaning the plastic cowl cover. Don't be stingy with it though, and make good use of some microfiber applicators or those yellow foam applicator pads Meguiar's sells.

I bought my 2005 Limited Roadster new just last July and the cowl cover, windshield header and other rubber trim was just a horrid mess. #39 did a great job on all of it, but since it's a cleaner only you'll want to use a dressing of some sort as a follow up. Whether you prefer 303, Meguiar's #40 or something else is up to you, but definitely put something on it.

If you're thinking will this help on the roadster deck lid tape - no. So far I have found NOTHING that will even out the appearance of the tape strip, especially since mine sat around for two years before I bought it. What have I tried:

Meguiar's M39
Meguiar's M40
Meguiar's All Purpose Cleaner
Black Wow from showcardetailing.com
Back to Black
303
50/50 water/alcohol mix
Simple Green
Orange Blast

I can minimze the two tone look for a while, but the texture even feels different between the exposed half and the half under the roof.