Probably a reasonable compromise. Those are very different focal lengths so you're not going to get one camera that's perfect for both.
It'll still be tough on the RC8, but it's likely to be a bit undersampled on the ED80. You could use the RC8 on the good nights, the ED80 on the bad nights, and maybe drizzle the ED80 on a typical night.
Someone has a mono SBIG 8300 (that Atik 383 you linked to is the colour version of the sensor) for $1600 at the moment. With the extra money you could put it towards a filter wheel and some filters and end up with a better rig.
http://www.iceinspace.com.au/forum/s...d.php?t=146551