You being 6'4" and me about 6"3' and a half -- we won't see that much different out the rear. Depending on how you adjust your seat, I can see the road about 4-5 feet behind me. [That would be a nice test to do tomorrow]. I can see good, and I have the lip spoiler form 3dCarbon.
As you said, with a larger spoiler, you may impair your vision some [more?], but it shouldn't be that bad. Usually spoilers are angled level to the sight of the eye, not up or down -- you should be able to lok over and under the wing.