3 cores sitting idle for compression sadly don't make your iphone faster, which has a fraction of the speed of a single core to decompress the data again, so while you would be able to push through more due to the reduced datasize, you would make the remote stutter because the cpu is only working on the image instead of the input too.

Normally you would actually want to look worse, so you get better input responsiveness, because in more intense games it can otherwise become very hard to test much through the remote actually.

To reduce the size, you can alter the corresponding define in the Unity Remote project.

To increase the quality of Unity remote, just open the UnityRemote project in Xcode, go to Classes>Setup.h

in there, you'll see a list f definitions:

#define IMAGE_320x448 1
#define IMAGE_192x384 0
#define IMAGE_192x320 0
#define IMAGE_192x256 0
#define IMAGE_160x256 0 // 1
#define IMAGE_160x240 0

I added a comment to the original setting and changed it to the highest setting which was 320x448. This gave me a much clear view of the screen. I couldn't even make out the buttons I was trying to test against on the iPad. Now I can see them ;)

I don't think there currently is a way to get better quality on that end. Might be worth a feature request on Unity iPhone feedback, though.

What I usually do most of the time is just use the iPhone for control and look at my screen. Of course, that's not ideal but it's "good enough" for me.