I've found a couple of things help when dealing with the huge file sizes of 360 degree footage and quality.
First of all make sure you are exporting using the best quality settings available to you. In Gp Fusion Studio this can be prores or cineform at 5.6K.
Then you need to edit the footage in Adobe Premier. I found that you should NOT use the Adobe presets (even though they have 4K and spherical video outputs for youtube). They are far too low a quality for good image quality. They will put the bit rate down to about 10 (where it should be 40 minimum). This can lead to blocking being visible. The higher the quality the better as youtube will attempt to compress your file again and as a result, badly rendered footage can look extremely blocky when uploaded.
Here's some samples of footage I did at different bit rate settings the latter would be at about the 40 mark. These are using the overcapture/reframe feature.
Also be aware of one more thing. When you look at your video on youtube it will detect your connection speed and automatically set playback quality based on that. You will want to ensure that in order to view your video at HQ, you make sure you select 4K quality when viewing your video.
