WHAt kind of distance can i expect with that LetroSonics system?
You can easily get the 500ft you need with a directional antenna setup. You way even be able to get 500ft with the included whip antennas, depending on the RF environment you are working in. The D4 system is 200mW which is at least double the power of any IEM on the market. Also TX sends two channels over the same carrier, so lower IMD than using (EDIT) a pair of IFB rigs.
EDIT: The D4 system operates up in the 900MHz band, which is pretty open, right now.
If you only need a single channel of audio, you can shoot it with a Lectrosonics IFB unit. Those put out 250mW.
If you want to email me I can see about getting a demo unit over.