Tire size's could do with advice

[daviesbike]

i am currently running 245 45 18 bridgestone potenza

i would like a better ride from the car can i fit 245 55 18 or maybe 245 50 18 to the rs6 wheels i currently have
what will the side affects be ?
how much will it affect the speedo how many mph will it be out + or -

the car is tracking all over the place as in it feels like a real effort at times to steer the car because it seems to want to follow all the ruts in the road and chase the white lines

also how would it be with the firm ish ride set up i have at the moment

many thanks