Would a high energy bottom quark 'decay' to a top quark?

It doesn't matter whether the $b$-quark is highly energetic, it can never decay to a top quark and a $W$-boson if it is on mass shell, by which I mean, $p^2=E^2 - \vec p^2 =m_b^2$. To see this, consider energy-momentum conservation, $$ b^\mu = W^\mu + t^\mu \Rightarrow m_b^2 = M_W^2 + m_t^2 + 2W\cdot t = M_W^2 + m_t^2 + 2 E_t M_W $$ However, since the energy $E_t$ is positive, energy-momentum cannot be conserved in the decay - the left-hand-side cannot equal the right-hand-side for the measured particle masses.

Now, if the $b$-quark is not on mass-shell, $p^2\neq m_b^2$, the decay is possible. An off-shell $b$-quark could be an internal line - a virtual particle - in a Feynman diagram. Decay widths (i.e. lifetimes) are different for particles that are not on mass shell. However, since particles off-mass shell are not propagating (they are internal in Feynman diagrams), when we talk of a life-time, we always mean an on-shell particle.