It's a negative, so a 1.5 stop over exposure is probably gonna be just fine. According to documents, it's not advisable to "pull" develop color negative film.

Here's and example of an ISO 400 film shot at ISO 100 (full 2-stop over exposure) and then developed normally (not mine): http://www.flickr.com/photos/foole/2105032322/

As you can see, there be no problems.