Myles,
I find that the internal engine is always flat, useful for many occasions. However, to perk it up:
1 - Render your view in both: internal engine and Open GL with all other settings equal (especially the pixel size of image).
2 - Open both in Photoshop, copy+paste Int Eng image into Open GL image.
3 - Ensure internal engine view is on top, set layer opacity to 50%.
4 - Save as to create a jpeg image and place in AC layout.
Voila, some sparkle returns, via the stronger whites that are available in the Open GL 3D method.
Snap.
PS change the layer blend mode to "multiply" for a slightly stronger effect. If the darks are the issue then try another blend mode.