Blip_Buffer provides treble and bass frequency equalization adjustment. In the original post, the Bode plot shows the default values. Below is flat equalization (bass = 1 Hz, trebel = 0 dB).
It's flat up to 16 kHz. From there, there is gradual attenuation until about 21 kHz, then it plunges to -25 dB by 26 kHz.
That's not bad. Above 26 kHz, it always stays around -25 dB. It would be nice if the attenuation were stronger. -25 dB is what you get when you pop in foam earplugs (though of course ear plugs attenuate all frequencies). It's a significant reduction, but it leaves things audible.
The plot above was produced using the blip_high_quality setting. Blow is the result of blip_low_quality.
This suggests it might be possible to further increase the cutoff steepness with greater performance cost. However, for efficiency, Blip_Buffer appears to fully operate with integer maths. The current high quality option may be pushing that design to the limits already; this is hinted by the fact that the quality constants appear to be precision bits (e.g. blip_high_quality = 16).
I did not test how performant Blip_Buffer is over the double-precision 13th-order Elliptic filter mentioned in the OP. But, assuming it is much more efficient, it's definitely a sensible option for emulation on low-end hardware given its filtering characteristics. On modern desktop hardware, better sound quality could be achieved via more advanced floating point filters.