Standard fin Koi should be the same as std carp in FCR if close in condition.
Koi are dumped weekly into grow out ponds during summer which is not exactly a farm pond setting so it's not going to be the same as a normal pond, in which case all of the BG are not going to be the same length, condition or population structure. Some will even be more aware of their surroundings than others.
I think this is a great project to take on, but in my opinion, it's going to take 3 different ponds with like numbers of fish with consideration to fish health of both Prey and Predator, cover and ambush points available.
LMB will eat when it wants to eat and we've all seen when one has bigger eyes than stomach. I don't think Bass need a huge mouth-full every time, but I do think a couple of things have to be present...1, high forage numbers that increase frequency of encounters with little effort to ingest, and healthy, stress-free conditions.
I believe you can get close to an assumption, but to know the true growth, we need to know the density of the fish being consumed at the time it is consumed, and how much energy did it take to run it down on that specific feeding burst. I "think" every encounter would be a different measure, although an average could be established if fish are sampled often but we know this is a stress inducer.
I'm sure there's a way to do it, just not sure how but there are folks doing this much smarter than me that may have an answer.