define tuning that you have had done?
The correct way of tuning/adjusting your 2C engine is
check timing belt slack - replace every 100,000 kms regardless of condition with new rollers and tensioner bearings
check valve clearances and adjust as necessary (replace valve seals while doing this job too as you need to take the camshaft out anyway) - tight valve clearances will cause intake backwash (coughing back to filter) and result in very bad economy and a very lazy engine that smokes too much.
check injection pump timing with dial gauge - (this is the final step in timing belt installation)
check for sticky/jammed/rusted CSD (cold start device - aka thermal advance) on pump
check injection pump internal pressure and adjust as necessary
check nozzle break pressure and spray pattern with very gentle and slow repeated strokes
check compression (after valve clearance adjustment)
After this correct procedure of adjustment - your engine will be back to factory specs and fuel consumption.
replace fuel and air filter with genuine toyota or some other very good branded part.
Print out this procedure from here and talk to a toyota service department regarding all the above. If they understand what Ive written then you will have a good car - if they dont, leave - find another one.
I used to adjust diesels like that and always had very very good results, a quick starting engine, very good economy and quiet performance.
btw these jobs dont require that you disassemble the head etc. The valve clearance shims can be replaced with head in place and valve seals too. Any good place worth their business would know how to.