There are ways too eek more power out of the 3.9... but it's expensive and takes time, parts, and labor. (and tuning) It would be easier and cheaper to make the LS handle better... than going fast in a straight line. I'd like to see that Buick take some of the curves I have taken.
Enjoy the LS for what it is... not what it isn't.
And if it was a 442... then it was an Oldsmobile Cutlass 7.5L (455 cubic inch).