I know the full poems of both 'an irish airman forsees his death' and 'leda and the swan'.
I also have knowledge of 'second coming', 'among school children', and 'easter 1916' and either know a stanza or a couple important lines out of each of them and the ideas and motifs behind each of the poems.
I dont think its essential to know all of the poems off by heart, just well enough that you can talk about them, or enough to use them to back up any technique etc within an essay or question.
There is no way that the BOS would expect up to know all of the poems well, but atleast 2 possibly 3 is essential. Enough that you can back up any statements you makewith examples.
I wouldnt be stressing about 'byzantium', its the hardest and not many people study it, so i highly doubt we will be asked about it specifically.
maddy