Thanks for your excellent reply.

So, in real terms - the SPL of headphones is something of a useless
stat?

Can the SPL of headphones be compared (assuming they've taken all at
1V, 1kHZ?)? Would comparing them only tell you that one will be
slightly louder than another (i.e. if one was 114, and one 111, then
like you say the 114 would play 2db louder at any given volume.)?