I have just done exactly this. HBO flights plus a separately booked car worked out cheaper than the combined flights plus car option, though I am pretty sure that the latter books you an Economy Plus fare, ie with checked luggage, so it is not a straight like-for-like comparison.

As an aside, BA boasts that it offers guaranteed savings on a flight plus car booking, compared to booking the two separately; I had a very curt exchange with BA about the fact that this was not the case when looking at Club Europe flights to Figari plus car last year. So arrogant were their responses, I gave up pursuing the matter.