You didn't specify, but it looks like you're running an ER-X.
With your speeds, you probably don't need hwnat. If you want to play around with Smart Queue (QoS) or Traffic Analysis you'll probably want to leave it disabled anyway. If you have no plans to use either of those features, you can safely turn hwnat back on. On a simple configuration, people have reported 385 Mbit/s with it disabled, and 935 Mbit/s with it enabled (see: beta forum). 385 Mbit/s is more than enough for your connection, so you should be fine either way.
I think the router requires a reboot after changing hwnat status, but I can't find the details. You might also lose the automatically created upnp rules when rebooting the router, so after the router is back up, restart those particular devices and they'll re-open the ports they need.
To enable:
configure set system offload hwnat enable commit save exitreboot
To disable:
configure set system offload hwnat disable commit save exitreboot