Can any algorithm efficiently be implemented as a SIMT problem?
No, efficiency cannot be strictly worse, for trivial reasons.  For instance, you can have all $n$ threads independently compute the same thing -- each doing the same thing that the sequential computation would have done.

This is not very interesting or useful in practice, for a number of reasons: (1) in practice, in SIMT architectures each individual compute element is typically less powerful than the compute element in a sequential system; (2) we care not just about compute time but about energy usage, and wasting energy on pointless computation would be, well, pointless.

At the same time, it is true that there are computational tasks that are inherently sequential: they cannot be sped up on a parallel processor (including a SIMT machine).  See, e.g., Which algorithms can not be parallelized?, https://crypto.stackexchange.com/q/606/351.