There's a rule of thumb saying that turbines start to make sense at 400BHP. I was told this is mostly a function of blade diameter as the main energy loss is due to the gap between the blade and the sheath and the larger the diameter, the smaller it is in relation to the whole thing.
One argument against turbines in single engine planes is that they stink and being placed in front of the cockpit, you get to breathe a lot of turbine exhaust. That alone would probably keep me from buying a single engine turbine.