Took a bit to find the right setting.

The yml file should look like

default:
  paths:
    features: 'features'
  extensions:
    Behat\MinkExtension\Extension:
      goutte:
        guzzle_parameters:
          curl.options:
            CURLOPT_PORT: 8000
      selenium2: ~
      base_url: 'http://localhost:8000'
      browser_name: safari

This way the goutte driver will use the correct port. Where as the base_url setting only seems to apply to that @javascript tag. Thanks to https://groups.google.com/forum/#!msg/behat/WCalw_TIjpo/FyaOjVfZP8IJ