Cool, another reason Qt is cool 
Ideally, though, I'd save non-8-bit color depths as a job for pixel shaders (that way SDL can handle it as well). GBE+ is going to have a GUI-less version (SDL only) and a GUI version (Qt+SDL), and I'd like to keep feature parity for both as close as I can. Though I'm not sure how much vividness you can expect from increased bit depths, since the values used in the end result are taken from the game's original 15-bit values. An unfiltered image will still give you only 32,767 distinct colors, until you start doing fancy blending that the GBA never did.
Ideally, though, I'd save non-8-bit color depths as a job for pixel shaders (that way SDL can handle it as well). GBE+ is going to have a GUI-less version (SDL only) and a GUI version (Qt+SDL), and I'd like to keep feature parity for both as close as I can. Though I'm not sure how much vividness you can expect from increased bit depths, since the values used in the end result are taken from the game's original 15-bit values. An unfiltered image will still give you only 32,767 distinct colors, until you start doing fancy blending that the GBA never did.