Hi Rich,
First, a clarification. The Unreal Wwise Integration casts rays using the unreal physics engine to drive the occlusion API in Wwise. The sound engine is then presented with the results (a number between 0-1, indicating how occluded a sound is). The ObstructionService on the Unreal side is generally seperate from Wwise Spatial Audio. This system predates Spatial Audio diffraction, and is generally (at least without a bit of hacking) not compatible with the diffraction system.
If you are looking to get better performance out of the ray casts in unreal, it might be best to do a bit of digging into the Unreal docs (or maybe someone more knowledgeable in this area can chime in). I suspect it would start with reviewing your collision channels, and the complexity of the collision geometry.
The only control in the Wwise Integration is OcclusionRefreshInterval, which can be set to a larger number to reduce the frequency of the ray casts.
Hope that helps,
Nate