Yeah that's what it comes down to, the transmission method. 5.8 will handle plenty of data, just like a wifi router but I don't know if there's a limitation on distance or if it's just a licence thing. I presume the cheaper one's you've been seeing claiming to transmit 1080 HD will do it but I don't know how well.
This is the stuff recently launched for FPV:
https://hobbyking.com/en_us/rosight-system-r-fcc.html?___store=en_us
https://hobbyking.com/en_us/ion-system-t-r-camer-fcc.html?___store=en_us
I didn't look into it as it was 100's of £'s for just the basic kit. If I'd have seen it reduced to what HK are selling it for now, I might have given it a go. Saying that, I don't know how well it works or the reception it has received so far. I don't think it's had mass market take up, most of us are still making do with 700tvl analogue images. As I say, the action camera I have records excellent 1080 video internally but the stuff recorded on the vrx is not that nice to watch back!