You can set Netflix to stream SD on any account (HD account just means you have the choice)
You can set video quality on the phone app.
You can also set globally it on your account page when you log into the NETFLIX site.
PS You also can't 'throw' the netflix image to another screen for free (will also use data)
I'm quite sure your app settings don't matter. As in, even if you have it set to the highest quality possible, Optus at a network level will limit it to 1.5Mbps which is SD so I wouldn't worry about that. Here's the FAQ I'm basing it on, specifically the line "If you stream through WiFi we won’t optimise your video." so to me they're saying we'll do stuff to your stream if your on our mobile network regardless of your settings but when you're not (i.e. WiFi) we won't touch it and Netflix will handle it.
A. Video will be streamed in standard definition up to 1.5Mbps. This speed is for a single stream on mobile and tablet devices, so you enjoy your favourite movies and TV shows when you’re out and about. You might get some pixilation for the first 30 seconds or so of your stream as your app calibrates to the network. To get the best experience, we recommend you use the Mobile TV streaming services mobile and tablet apps and set the quality to “Auto”.
If you stream through WiFi we won’t optimise your video. The quality will vary depending on the app settings and your WiFi internet service provider. This may use data from your home internet plan.
You said you're "watching your data be used". How? My Optus App? Data usage meter on your phone? Usage SMS alerts?
Can you link to the plan you signed up to?
That's surprising to me. I guess it explains why the service is Opt In rather than just activated automatically (I always wondered why anybody wouldn't want free streaming data - but forcing you to SD quality obviously doesn't appeal to some users)