Maybe your system is not fast enough to provide higher speeds than 8x.
i've read conflicting threads about lowering the speed as far as quality and that for "good media" to go with the rated speed.
Rule of thumb: 12x is normally okay for 16x (max) media, sometimes, results are better at 8x. 8x (max) media normally can be used at 8x without problems.
what is the lifespan of a burner?
It depends. Nobody can tell you.
i notice less coasters if i turn my computer off for like an hour and then try again.
Then there might be some general issues with your system. Maybe bad PSU, bad motherboard, high temperatures or similar.
i downloaded the cdspeed program but am not sure if i'm using it right.
There is an excellent Users Guide in the "Blank Media" section.
does it tell the "sweet spot" on a blank dvd or just quality on a burned dvd?
The latter. Read the Users Guide, and you know.
i have a benq dw1650 and a hl-dt-st gca-4164b. I'm not sure how to update the firmware if that's the issue,
For your Benq drive, Read the FAQ in the Benq Drives section. You LG (HL-DT-ST) is an OEM drive. No firmware from LG available. If the drive was preinstalled in your computer, there might be newer firmware at the computer manufacturer's website.
but i went into the windows device manager and checked for driver updates and there weren't any...
That's normal. There is no such thing like downloadable drivers for optical drives. A driver is something very different from a firmware update. So, your OS doesn't care about.