Tires

245/40/19 upfront and the 255/40/19 on the rear you say.
A tyres size works as follows; 255 are the width of the tyre. 40 are the height or profile of the tyre and 19 is the radios of the rim.
It is perfectly fine to use the tyre set up you have at the moment because its rear wheel drive and you want a wider track for grip under acceleration and braking. Upfront you a wider tyre increases tyre roll when the car is pushed in to a corner so a narrower size I better. The different tyre size is ok just as long as on a two-wheel drive car you always use the exact same size tyre on each axel.
I personally think this is a point less exercise! You will never feel the difference of having this tyre setup with normal driving conditions or Evan a track day.
When theirs tyres where out I would fit the 255/40/19's all around. Its good to have them interchangeable and it more comfortable.
Remember that if you are buying new tyres always change them in pears.