Yes there is a defiante noticeable difference, I just got another 99 PCM retuned for me. One thing I have found, although it may just be on mine, make sure you get your timing tables smoothed out and have a little extra fuel added. I was getting alot of extra ping on my first couple tunes, might have been running a bit lean.
I am switching to the 02 PCM when I get my headers in mid June or July, it is supposed to be able to take more timing and speed the truck up a bit.
Chris
2009 Silverado, HPTuners, all stock.