I think the Tamiya America website lists the number of different parts.
At the end of the day it is A LOT. It would cost a huge amount to upgrade a normal TA05 to R spec, a lot more than the relatively small difference in price between the two kits.
TA05R is virtually a completely different car to a basic TA05. Shocks, suspension, most of the drivetrain, most of the hardware is different, even the plastics are different. About the only parts that are identical are the tub chassis and bumper.