Absolutely no idea about the oil leakage rate Paul, my hunch is that it would be proportional through the breakdown process - but it's just a hunch.
Krill pellets - not something I've used.
Impact of high oil trout/halibut/salmon pellets - there are three main reasons I've stopped using them. Firstly, years ago when I worked on a fishery/fish farm my boss and his wife who were both highly qualified aquaculturalists always had a bee-in-their bonnet about them and wouldn't allow there use in their fishery. They were adament they were bad for the liver and kidney systems of the fish. Obviously they never used for growing on fish, using more expensive low oil 'coarse' pellets. That was about 25 years ago, but it's always stuck with me, so when Inhave used pellets in the past it always been in low quantities. I understand that this has been subsequently backed up by Studies etc, I'm sure you'll find a plethora of stuff if you go digging.
Secondly, given all the health warnings surrounding eating farmed fish, especially salmon, e.g. high levels of dioxins which are proven carcinogens, that sets alarm bells ringing with me. No such health issues are concerned with eating wild fish...make of that what you will. I know we don't eat barbel (I hope..), but I'm sure you get my point.
Thirdly, our oceans are in a parlous state, and over fishing is a major issue. Harvesting fish to feed to fish just for pleasure angling seems unsustainable to be..each to their own but it's not for me. Hence I've stopped using fishmeals full stop in my baits. This is the major reason for me.
Another issue is also digestibility. Apart from a trip or two to the Havon in summer, nearly all my barbel fishing is done between Nov and the end of the season. Having an oily bait that fish struggle to digest is of no use to me in those conditions.
Oh and finally, I really don't miss the rank smell of pellets lingering on my hands!