I think the stock firmware must have been G7H9. Mine was manufactured in Sept 2004, and sold to me late December, and it had G7H9. This firmware was known to be unable to report PIF. When I flashed to G7P9, it reported PIF for the first time.
What version of CD-DVD Speed are you using? A new one was released a few weeks ago, and may behave differently with the various firmware, but it works correctly for me with G7P9, which should be functionally equivalent to B7P9.
G7Z9 is reported to be older than G7P9, dispite its recent release. Search for G7Z9 and you'll find some discussion on it.
Now, 6 minutes to burn a full DVD-5 (4.7 Gbytes) is about right for 16x. In fact, you'll find there's an "under 6 minute" thread here for proud posters that break the 6 minute barrier. 8x should take around 10 to 12 minutes, and 4x will take about 20, give or take the walking power logic's activity.
It's good that G7Z9 seemed to work for you, but that may bring comments from members more experienced than I about flash problems with your eariler attempts. My one and only flash was to G7P9, and it had no problems, but others have reported that reflashing the same firmware improved results, chalking that up to a bad flash.
There's also a lot to be said for conflicts with IDE drivers, and VSO software which installs a "patton couffin" driver that's known to cause problems.
Now, when you said you tested at 8x and it runs at 1.6x, this is the test speed not the write speed, correct? That may be the media, but might be configuration issues. Check your burst rate to verify that's around 23 Mbytes per second. However, for some media I've read and noticed that it must run slowly to read what it can. I had a few media that scanned at 0.8x, but many others reach 8x.
What surprises me is that B7P9 didn't work well for you.
The standard questions I'm sure would be posed include:
What cable, and what position is the drive in, and what is it sharing with, if anything?
Was media in the drive during the flash?
Did you flash in safe mode?
What media are you using?
What software are you using to burn?