An option to override batch size?

Issue #3067165 on drupal.org by scoff.

Currently, batch_limit for drush migrate-import is 1/100 of total items to process. It's fine (or doesn't matter) for sources like SQL but for fairly large file-based (xml in my case) sources it seems to slow down the process with each new batch. First 1-3 batches of 25k items import almost instantly, then it starts to slow down considerably. No big deal for a one-time process but in my case it's a recurrent (daily) migration. The source file isn't large so I think increasing batch_limit would help.

Is there a way other than hacking the code?