As for your enclosure, I think 2.8 cu ft net tuned to 28 Hz is a good place to be. Consider the attached graph.
The solid magenta line represents the flattest frequency response at 3.16 cu ft net tuned to 23.4 Hz. For a home theater system, I think this would be a smashing setup!.. but for in-car, the effects of cabin gain make it so that we needn't tune so low, since lower frequencies will be boosted naturally. By tuning so low, you might notice that the higher frequency output is not as pronounced as you would like it to be.
The dotted green line in the graph represents your proposed 2.8 cu ft net @ 28 Hz. An excellent alignment for in-car use in my opinion. Should get nice and low, and maintain some hefty output over its most common frequency range.
The red dashed line represents 2.5 cu ft net @ 32 Hz. This is also suitable for in-car performance. It will offer you heightened SPL levels, but your low-frequency extension will not be as nice as the enclosure you have proposed, and overall I would believe this alignment to be a little peaky (which some people may prefer).
So there you have it. I'd say stick with your gut instinct and roll with the 2.8 @ 28 Hz //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif