All ASIC for Bitcoin do a double SHA256 hash on an arbitrary 64 byte long string. But I believe it might be possible to design Bitcoin ASIC that allows also computing only single SHA256 hash.

Firmware would decide if ASIC is returning SHA256(SHA256(x)) or SHA256(x) value. It would allow to create much more versatile devices.

Question is, did any of current ASIC manufacturers done it already? Would such design be much less efficient (J/Gh) and have lower performance (Gh/s)?