Quick update:

I had a new straight pipe welded in place of the flex pipe, and finally put in fresh fuel (it near empty and still on the diesel from a few years ago).

I downloaded an app called "Drag Racer" to try and get more accurate results (versus looking at a stopwatch when I see the speedo at 60). I measured 15.9 seconds 0-60.

I then put in a new air filter, and made sure all the ducting to the turbo is clear and that the air valve wasn't stuck. I then measured the 0-60 again: 15.2 seconds.

Tough to say if it's due entirely to the air filter, maybe to the seafoam doing its job, or maybe one of a thousand other variables but regardless, I'm happy with the progress.

On to the ALDA and fuel filters...