Personally I don't rate spirulina, but then I have a few mates who know a thing or two about bait that do.. split decision on that one..
Bait formulation, when talking about this 'digestability' thing is wholly reliant on how much a carp can utilise.. and that depends on the first limiting amino acid..Ultimate protein content is of no relevance..
The aquaculture? meals in use still need supplementing, or at least combining with other protein sources that can give adequate supplies of whatever

amino is missing from them for a carp.. let's not forget that whatever fishmeal you use, with the exception of one.. (anyone want to hazard a guess at this..??) you will have a deficiency of one or more aminos.........
Koi feeds are surprisingly low tech.. they just cost a lot..