the tiptronic will alow you to gain 10km/h b4 you go into the next gear compare to flooring it.
however, it will drop you to a slighty worse powerband in the GTS than flooring it.
you do the math.
about the gas saving question... it depends. if you are on an uphill, and you shift at 2200 from 3rd to 4th, the power will not be there for you. in other words, you will waste gas because you are injecting more gas and no power is given from the engine because you on in 4th. it all depends on experience.