There are two products out there that I know of that allow you to use a gravity feed gun at any angle. One is the Devilbiss DeKups system,
http://www.autorefinishdevilbiss.com/products.php?pg=149 and the other is the 3M PPS system,
http://solutions.3m.com/wps/portal/3M/en_US/Marine/Home/Solutions/PPS/Both use a disposable cup liner that collapses as you paint, allowing you to paint at any angle. ( Kind of like those baby bottles. )
As for the question of which is better, gravity or siphon? There are a million painters with a million opinions. I think it may be more a matter of preference than performance for some. I have both and use them for different situations.
For flat and vertical work on large pieces, I like my suction guns because they have a larger cup and I dont have to fill them as often. The downside is, because they have bigger cups, they weigh more full and can make my arm tired quicker.
For smaller jobs or ones with a lot of overhead spraying, I use the gravity guns more often. My gravity guns are not HVLP, so I can't speak to the overspray issues that those types of guns lessen.