Export times won't improve much from 8GB to 16GB.

That’s not where the bottleneck is. Lightroom export is heavy on CPU and disk I/O, not RAM. You’ll see a bigger speed bump from a faster CPU or an SSD than from doubling memory. For light editing, 8GB is enough to keep the catalog and a few raw files in memory. Export processes aren’t particularly memory-hungry once the data is loaded.

Where 16GB does help: general responsiveness during editing, especially if you keep Photoshop, Chrome, or Spotify open at the same time. But if export speed is your only concern, save the money and upgrade the processor or storage first.