I did some testing with this yesterday, and can confirm that they are stacking, but also noted something else that's pretty important. It seems as though sneaking has a speed limit of roughly 150% normal speed (guild buff + VIP bonus for instance would give the same bonus to any non-sneaking toon). For instance sprint boost didn't seem to apply at all when sneaking at super high speed. This means that while this stacking certainly is useful, it requires a pretty hefty AP requirement (16 AP needs to be spent between 3 trees in 3 specific classes to unlock it), and best case scenario is roughly as fast as a rogue/bard 20 with striding 30% and their class based speed buffs.