Yes the engine is turbocharged. Without going to deep into the weeds here: roughly speaking the TIT at say 18" MP on final might be around 1200-1300dF (it's been a while since I've flown one). On a correctly set up J2B, the TIT at full power should be: between 1200-1300dF!
Ummm....whilst it has been over 13 years since I last flew a Chieftain, and I only had about 40 or so Command hours in them (s***loads in the RHS on twin Pilot ops) the Company I worked for had it 'chiselled in stone' on the Ops Manual that on the downwind leg power was set at 25''MAP 2300RPM. The first stage of flap was lowered (max 154kt IAS) which reduced IAS to below the lowest gear extension speed, which was 126kt for two of the a/c in the fleet. The other two a/c had higher extension speeds of 156kt.
MAP remained unaltered, and was not reduced until short final.
The above quoted from my copy of the Chieftain PIM.