Tires: Upsizing, sway bars, directional stability

Question
I have a vehicle with stock tyres 175/80 14 .I wish to upgrade it as there is bodyroll on cornering. The sizes i do consider are 185/80 14 and 195/70 14( can get 185/75 and 195/75 also) and there is another plan to put wider tyres in rear to get better control and understeering while cornering. What are the effects on performance,stability and mileage if these sizes are woked out in the vehicle ?

Answer
If you want to reduce body roll, springs and sway bars are much more effective than tires.  

But tires do have an effect on cornering capability.  Needless to say, wider is better, but all these things are going in the opposite direction of mileage.

Using the same tire all around maintains the directional stability at the current level, and as you point out, putting wider tires on the rear will increase understeer.