The following answer I received a few months ago from James Wang, a technical marketing analyst from NVIDIA:
Q: In the GeForce family, double-precision throughput has been reduced to 25% of the full design. Was this decision made to discourage the use of these products for professional use (where Quadro and Tesla are targeted?) Considering the fused support of single- and double-precision calculations in the CUDA cores, how was this change even applied?
A: Yes, full-speed double precision performance is a feature we reserve for our professional customers. Consumer applications have little use for double precision, so this does not really affect GeForce users. Having differentiated features and pricing is actually fairer for all. Given the option of enabling all professional features on GeForce and having gamers pay for them, or disabling them on GeForce and offering a more compelling price, we feel the latter is the better choice.
Regarding the second part of the question, the architecture is designed to support this kind of configuration.