Fluidsynth can't do time stretching of samples. And I must admit I have a hard time imagining how a feature like that would fit into Fluidsynths synthesis model...
In my experience, time stretching is heavily dependent on the type of sound you want to stretch. Voice needs different algorithms and/or parameters than percussive sounds, for example. I've played around with rubberband on Linux quite a bit and noticed that the many different parameters you can set make a huge difference, and that that difference heavily depends on the kind of sound I tried to stretch.
Maybe you could stretch your samples with an external tool like rubberband in many different speeds, then create a soundfont that has your samples assigned to different MIDI notes.
But you state that you value the accurate reproduction of your samples. My guess is that as soon as you stretch them, that accuracy goes away. A 4/4 base drum rhythm at 100bpm time stretched to 50bpm will sound quite different than a base drum sample triggered separately at higher speed.