Allow performance details to be routed elsewhere without writing to the database

hmmm... Implementing a new hook wouldn't cut it I think... Adding some sort of setting that allows you to write a custom function containing your logic to store the data. The function is passed an object containing the data and you do with it as you please.

It could be as simple as:

  if (variable_get('performance_detail', 0)) {
    // There used to be a module_invoke_all('performance', 'data') call here. As
    // it was undocumented and therefore unknown, it has been removed. The data
    // column has been kept so that we can re-implement if needed.
    $params['data'] = NULL;
    $function = variable_get('performance_detail_store', 'performance_log_details');
    call_user_func($function, $params);
  }

Would need logic to read the data back again to fill the detail page, that's a different story...

The syslog vs dblog is implemented using a hook:

    // Call the logging hooks to log/process the message
    foreach (module_implements('watchdog') as $module) {
      module_invoke($module, 'watchdog', $log_entry);
    }

Although it's the Drupal way of doing things, I'm not sure it's suitable in the performance module. I'm afraid that this will have quite an impact especially for sites with lots of modules, hence my previously proposed approach.

I would like to be as Drupal standard as possible though... And using a hook is nice and clean...

Edit: but when detailed logging is enabled and we would use a hook approach like hook_watchdog, it would be triggered on every page load which I simply do not find acceptable...

Patch applied, modified a bit and backported to D6.

Available in next release.