The sound libraries for Spectrasonics instruments are recorded at the sample-rate best suited to the source sound.
Do Spectrasonics instruments sound better at higher sample rates?
That would depend on your audio hardware and software configuration. In most cases there won't be a difference in how the instruments sound at higher sample rates.
NOTE: Some Omnisphere 2 effects, accessible while using Keyscape integrated within Omnisphere 2, may not sound correct at higher sample rates. If you are using a patch or multi that exhibits issues when applying effects, it's recommended that you set your sample rate to 44.1/48 kHz.