Have you looked at the thumbnail images that the Kobo creates. You can find them in the KOBO directory under IMAGES. You will need an image browser that ignores extensions and reads images according to the file headers. I use XNVIEW which is free.
When the KOBO processes a book it creates 4 image files that it uses for sleep/power covers, Library thumbnail, etc.
They are sized(in pixels), 355x530, 149x223, 60x90, and 120x186.
If you check the proportions of your 800x600 cover, you will find it does not match 530x355. It seems that when it processes the cover images, if they do not match the 530x355 proportion exactly, they are slightly shrunk and therefore you end up with small white margins.
You might try editing the cover to the correct proportions and see what happens. I would recomend 1060x710. That way you start from higher resolution and if you get an eReader with a higher res screen later, the cover will still look good.
Also, you can actually edit the KOBO images by hand. As a test I have replaced a 355x530 LARGE image by a 600x800 image and the KOBO showed it fine. However, you would have to hand replace them whenever you do a Factory Reset since the KOBO would reprocess all your books again.