The picture quality "should" be identical on the 80,81 and 85 models as they have the same spec panel,800 has a higher spec panel so might be different.The performance/quality of the different sizes can sometimes vary though, ie a 42" can get a 5star review where as a 46" for example only gets 3 or 4 stars.
The biggest difference is the bells and whistles on each model. 80 is the basic, 81 has freesat and some features of the 800, 85 has improved audio and IFC and the 800 has the full monty of features but no freesat.
I went for a 42"PZ85 as I wanted the improved audio for late night and normal tv viewing. I do have a proper surround system for movies though.
Best thing you can do is to set it up properly with the THX optimiser, or you can buy
this on dvd/blu-ray which is even better. I also bought one of
these for £24 delivered from amazon, much cleaner picture and less picture noise.
If your not interested in the freesat I would choose between the 80 and 85 model. I don't think the extra expense of the 800 is worth the money for the slightly better picture.