Hi Paul
i also have moved from the ct to an IS i have the luxury model at the moment i am
returning 46.7mpg and during the warmer weather i get 52'ish mpg so for me
its only about 6mpg less than the ct its 6mpg worth losing,the IS
is quicker ,quieter and smoother and pairing your phone is easy then transfer your phonebook in 1
what more can you ask for,i am on 17" all round and the tyres cost less
for the IS compared to the CT must be a more everyday size .
so to sum up pick up your wallet have a test drive then buy one before leaving the dealership.