2.5L V6 is a entry model engine
australia had the entry model is250 only for years before the is350 came
GS250 Also a entry model
it's to compete with BMW and Audi
if you look on the specs
BMW320
Audi 1.8T
MercC200
instead of making higher spec models like bmw 323 325 328 330 335 etc
lexus chose the option of adding more standard options
People generally who buy is250's *new* are gentle and dont give a two cents about speed so for the main IS buyers market the 2.5L engine is perfect
lazy at speed but enough power to not a fool out of itself
bmw has probably sold more 320i's over 335 or 330's
like
reason for this higher fuel needing is due the compression ratio which is in the high side of 12.X.X ratio
whilist the IS350 or the Aurion with the 3.5V6 is at a lower 11.X.X ratio
Infact the 2.5L is not underpowered
biggest problem of the car is the weight
For instance Honda accord Euro is a 2.4L motor makes 147kw
the 350z with a 3.7L makes 230kw or so reason for this it's purpose is a sports coupe
unlike the IS250 it's a entry model luxury small saloon
I know u dont want to hear this but
to fix this problem lexus did bring out the IS350 back in 2006 to cater for the sports luxury market its just australia didnt receive the is350 till 2011