Previews look poor on Apple TV

What do you have your Aperture->Preferences->Previews settings set to?

The Photo Preview and Photo Preview quality settings will both affect how the images look on your TV

Remember if you make any changes to these settings they will only affect new previews changing these settings will changes the current previews. To have the current previews use the new settings you will have to re-generate the current previews.

I would set Don't Limit and then play around with the quality slider.

Remember for the new settings to take effect you need to recreate the previews. Also the new previews are going to take up more disk space so keep that in mind. Depending on your availiable disk space you may just want to use the higher quality settings on images you intend to display on your TV,

But that means you end up with a massive Aperture library. Not a very elegant solution...

If you choose Don't Limit I'm assuming that means the Preview size will be the same as the image?

If Apple TV is only 1280 x 720, then why not set it for something slightly larger? To allow pan and zoom. Or just setting it to 2560 x 2560, maximum for a 23" LED display and large enough to handle pan and zoom in 1920 x 1080, should ATV at some point come out with full 1080.

Keeping it at Don't Limit (if I understand the feature) could create some pretty big images, if you are using a more recent camera that has fairly high pixel count. 10 MP cameras are about 2x the resolution of 1080 HD.

Also, I think you can drop the quality a fair amount and not notice any degredation, at least on ATV. Doesn't the jpeg compression start to degrad most noticably with dark images, or fine gradients? You could probably get away with a Quality setting of 7 or 8 and be happy. Of course, you'd have to test this out with your own images, ones that would show the most issues when lowering the Quality setting.

To keep the Aperture library size down, change the preview size of just the images you want to display on the AppleTV.

Change your preview settings to Don't Limit, select all the images you want to display on the AppleTV, go to Photos>Update Previews and hold down Option so it changes to "Generate Previews". Click on this and the selected previews will update using the "Don't Limit" preference.

Once this is completed go back to preferences and change your preview size setting back to whatever you originaly had it at.

There are lots of ways you can select just the images you want to upgrade, you could perhaps use colour labels, you could regenerate the previews for all 5 star images.