The reason Maxwell doesn't take a hit is because NVIDIA has explictly disabled async compute in Maxwell drivers. So no matter how much we pile things to the queues, they cannot be set to run asynchronously because the driver says "no, I can't do that".
NV driver runs asynchronous tasks in one queue on Maxwell, similar to if they were submitted in one queue ("async off" in Time Spy). If NVIDIA enables Async Compute in the drivers on Maxwell, Time Spy will start using it. Performance gain or loss depends on the hardware & drivers.
Ultimately some AMD cards gain quite a bit (ie. they have a lot of shader units idling while rendering and they are very good at using them for the available paraller loads). Some AMD cards gain less or not at all (either less capable at paralleriziing, less idle shader units or no idle shader units at all - for example a HD 7970 is hard pressed to have any to "spare")
Some NVIDIA cards cannot do this at all. The driver simply says "hold your horses, we'll do this nicely in order". Some NVIDIA cards can do some of it. They might use another way than AMD (more driver/software based), but the end result is the same - the card hardware is capable of doing more through some intelligent juggling of the work.
