Yes, they are still ideal for phase purposes.
The beautiful thing about lack of reflections (there are still A lot) is that you get less comb filtering.
Also, you should get the harley rockford setup...I heard that was tuned by someone really good and really into car audio....maybe even enough to make a helpful spreadsheet or sitting on a harley in their profile picture on this site
Open air will also have issues with cancellation when driving due to environment, so road noise will impact stuff more and drown things out. Ideal if you can compensate for this some way...like the rockford harley stuff does.