i think the reason they used the supercharged V8 instead of the TT V12 is weight, i have no numbers to go by so i could be 100% wrong. but i am assuming that the V12 would be heavier and since its HP output is about the same the SLR doesnt need all the torque being lighter than the CL S and SL 65's. (not that any car really needs 738 ft-lbs

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so they were more concerned to keep the weight down in certain areas. plus its a racing car when driven as such, the revs will be kept in the higher RPMs and the torque from 2-4k rpm will not really be used all that much.
i think i answered your question.