I got the Powerlung last Christmas and within several months I increased my static from 3:30 to 4:15. Not sure if it was exclusively the Powerlung or just additional practice, but it certainly didn't hurt.
The past few months, I have been less consistent using the device and it does seem that my statics have decreased - although I quit pushing myself in this area as I was always alone in my hottub when practicing.
The only equation that I know worked for me: consistent use of the Powerlung combined with frequent (once every week or two) static practicing equaled increased times. Which of the two variables is the more important one, I know not. However, it did seem that I was stuck in the 3:30 range until I started using the Powerlung.
Scott
P.S. thin_air, the Powerlung (two models now) is available at numerous on-line shops. Just do a word search and then find the best price (with shipping) that you can. I bought mine from a Florida dive shop and they shipped it to me free.