I noticed that along with their data logging capability, it also prints out your hp, how accurate would this be without using a dyno and just getting the numbers directly from the ecu? I dont see alot of s2k drivers have this on their car, but the website is www.plxdevices.com...

 

I've got a plx 500. It doesn't really provide a dyno-like log that you can use, but will display hp/tq curves (and peak) on the LCD display of the unit.

I think it's relatively accurate IF you setup your pull right. Meaning, on as flat of a ground as possible AND doing a pull in each direction and averaging the two results (to eliminate and distortion due to slight inclines in the road).