In what was effectively still air I have had 173kts TAS at FL180 in a turbo arrow. This was with an engine that was condemned several hours later. There will be little difference with cruise figures at this altitude between a Turbo IV and a Turbo III. Full throttle altitude is generally reached in UK air densities around 12000 feet.
I have saved 45 minutes flying time from Scotland to the south of England catching tailwinds and enjoying ground speeds in excess of 200kts, in smooth air above the weather.
If the original poster wants to cruise around at lower levels the normally aspirated version is a better bet, although good rates of climb in a turbo can be maintained much longer when mountainous areas or IMC present an obstacle. The Continental turbo engine does not suffer rough handling as well as its Lycoming IO-360 counterpart so is not the type of aeroplane to rent out to other pilots without absolute confidence in their abilities.