Granted, most of what Bostrom refers to in context of the biological path is mostly related to human augmentation, genetics, selective breeding - things of that nature. What I'm referring to is biology serving as a raw computational substrate.
While biology (or at least brain tissue) is dramatically slower in terms of raw latency when compared to microprocessors, it's arguably a far cheaper and vastly more dense form of computation. It also has the natural algorithm for intelligence that we keep trying to deduce and transpose into silicon - well, at least the raw form of it - built in by default.
Assuming ethics are thrown out the window and human brain tissue is grown in vitro at scale, then it probably would make sense to hook it up to a supercomputer for good measure. At the very least, we have the technological foundations[1][2] for such an experiment at present day, it's just a matter of scaling things up.
If I were to hazard a guess, human brain tissue grown to significant scale would probably not magically achieve sentience, or exhibit any complex anthropomorphic traits. On the contrary, it would probably have more in common with a simple neural network implemented on silicon, at least in terms of its capacity for self-awareness. Conversely, it would stand to reason that an entity with a higher degree of self-awareness, implemented in silicon, should rank higher in terms of ethical considerations than living tissue.
Obviously the aforementioned experiment would be completely unethical, but it's interesting to ponder it as a hypothetical - that today we may have the capability to bootstrap a superintelligent machine using biology as a computational shortcut. But we can't, because ethics. Instead, we're waiting for the inevitable increase in computational power to arrive so that we can do essentially the same thing, just in digital form.
[1] http://www.nytimes.com/2014/10/13/science/researchers-replic...
[2] http://neural.bme.ufl.edu/page12/page1/assets/NeuroFlght2.pd...