Well, it actually really depends on the type of calculations that a certain filter is going to do; after all, GPU are good for certain types of calculations and "bad" for some others. For instance, consumer grade GPUs have 16bit and 32bit capable units for Half precision and Single Precision Floating Point arithmetic, but they don't have many 64bit capable units. Besides, depending on which GPU you are using, it may not achieve fast enough results for certain types of calculations, no matter what. I do agree that having both the option (CPU or GPU) to use a filter should be the way, though.
As to f3kdb, give it a shot.
The two most famous debanding filters are in fact f3kdb and gradfun. They are not terribly slow at 8bit YV12 planar, so you might be lucky and get something nice and smooth as real time output for low resolution sources and maybe even HD. (Not sure about FULL HD sources and definitely not 4K).
Last but not least: using filters to adjust things on playback can be fine, but I do think that an encoding is supposed to be done in a good way in the first place and as long as you are gonna watch official sources like Blu-ray, it's actually unlikely to suffer so terribly from banding that it's unwatchable (as it's still H.264 yv12 8bit, but with about 40 Mbit/s there's plenty of bitrate to avoid terribly wrong looking artifacts). I can't say the same thing for DVDs (in which the MPEG-2 sometimes caused some pretty bad results) and TV channels where MPEG-2 and the very low bitrate available (compared to Blu-rays etc) actually creates some unpleasant artifacts. Anyway, nowadays we are heading towards H.264 for HD and FULL HD 8bit contents and H.265 10bit for UHD contents in TV channels as well and although the bitrate they use it's not so high, newer codecs are still gonna act as damage limitation and the user experience shouldn't be bad.
In a nutshell: try f3kdb and gradfun yourself, play with the settings, cross your fingers and maybe, just maybe you'll find the sweet spot you are looking for. If not, just stick with madvr.
