PowerVR chips are really not that brilliant at GPGPU. In shaders, 128-bit vector operations are executed as four independent 32-bit scalar ops, for instance. And memory bandwidth is really limited.
PowerVR is infamous for being "the worst graphics core ever". That's true regardless of what silicon it's on - Broadcom, TI, or even Intel. Look up "Poulsbo" and see the flood of complaints for terrible drivers.
My Cedarview Atom was affected by that issue, so initially, I stuck in an old FX5200 card to get an acceptable level of performance. Now I installed some better drivers so I no longer need that FX5200, and I get great 2D performance and good video performance (it offloads but nowhere as well as Nvidia's VDPAU), but try to start any 3D program and it won't work. (Fortunately, the Atom system is not for gaming.)
As for Broadcom wireless chipsets, they just can't compare to the good brands like Atheros/Qualcomm, Intel, or Ralink. But if you have one that is supported by the open source driver, unlocking it to use extended channels is supposed to be easier than for the others.