Are there high flow floor heat registers available?
There are various sellers of louvered and non-louvered floor vents.  These can be pricey though.

Of course if you're going to be replacing the existing cover anyway, you may as well try the other poster's solution of just removing the plastic from the one you've already got.

You titled your question 'high flow'; the above I'd consider more 'normal flow'.  For people looking for increased airflow from a vent, there are also powered vent fans available.  These aren't decorative and are more expensive.

For a custom/fancy outlet cover with high flow via fan boosting, I don't know of any products on the market, but the fans are essentially like computer fans you can buy for $5-10 plus a power supply and an airflow sensor/switch.  Could make a great DIY project.