I think for a C182, the best upgrade is the 260HP IO540 STC.
The parallel valve 540 is probably the most reliable of the entire fleet, you get enough extra HP to transform the aircraft and I guess (too lazy to look) less weight than the 550.
There is one in Sydney and its been an outstanding success.
I know I always say too much HP is almost enough, but really the 260 is probably the better balance of HP for the airframe.
Happy to be proven wrong though if someone has some real world data.