I'm not the first to mention this.
Most recently, NZCROSS in the "srt6 performance" thread said

"Anyway, if it is anything like mine ( also purchased brand new ) then wait till you get around 8k on the clock, they seem to loosen up and really start pulling hard - lock it in 1st and turn the traction control off, stamp on the gas and hang on - its addictive so be warned "

I've heard it elsewhere too.