I have an 05 6oosdi gtx with the same issues. Just have to keep track of miles and fill up at any opportunity.
I just picked up 2 brand new 09 500ss they now come with electronic gauges in the speedometer.
There is a post on Dootalk on how to fix the gauge. Easy to do Pull it our of tank and trim off 1/16" from the end. The rod is too long and pushes the gauge ball up against the viewing plastic.
I have read dyno reports on this 600 cc motor that indicate is has from around 109 hp to as high as 114 hp. Depends on air temp, engine temp, etc., but a good overall estimate is 110 hp for this motor.