Hard to say. If different authors are coding each plugin you have to take that into consideration that these current strengths and weaknesses of a particular plugin may not solely come from a strength or weakness of the particular API (aka dx or OGL). If its single author he may be more familiar with certain particulars of a specific API. (haven't been following dolphin much) (isn't zerofrog coding a plugin?)
Remember these plugins are still work in progress so there are many bugs still to be ironed out. So i wouldn't use the current state of the plugins to determine if one API is better than another at a particular thing. So its hard to say at the moment, it could be numerous bugs exist. Or a portion of the code could need optimizing. Of course I could be wrong but thats something you have to take into consideration.
LOL, with the projection hack on, now I can see the backdrops, but not the menus.
The weird thing is, with OpenGL, at first, I can only see menus, then after having a battle and going back to the menus, the backdrops show to, so why cant they show from the beginning ?
Projection is like hum you are at position and see from a point (0;0;0) and with your eyes you can see until point like z coordonate like 70 pixel in front of you. So after 70 pixel you can't see other thing and all things behind you. So it's like a vision of a person. I never use DirectX but when i check to see how it was made in DirectX to adapt it to opengl and change opengl projection to make a real good Projection and not a hack (they are just different start position and end position. They are just different point of you) which will work all the time. But DirectX is totally different, it manage it himself instead of OpenGL that you must write in your hand all the value with some calcul (much long in OpenGL than DirectX).
So, i have Pokemon Revolution, and Hack Projection is not for me a good idea because this game need two projection in a same time to work with all graphics. So now, we have the choice to be able to use many projection in same time (more projections, more slower. Example : 2 projection need 2 display so ^^ ...) or they must find the real good projection and not an approximative one. I know that it is difficult to find it but i think with their skills they can find a good one (maybe threat a Nintendo Developper to know what they use? ).
Finally if Games DEveloppers use different type of projection in same time, they are really ... you can understand .