You are right pharnorth, sc is usually absorbed slower than im because muscles have more blood supply than the loose tissue and space under the skin.
I am not sure about efficacy, I would have thought im injection by mistake could be more common than you might think, particularly commercial farmers who are injecting large numbers, the gun may well be used deeper in some than others.
I would expect that with im injection there is more chance of the "vaccine reaction" swelling because by its very nature, vaccine stimulates the immune system and hence causes inflammation, often evidenced by swelling. If injected into muscle with less space to dissipate the swelling and a bigger blood supply to create the inflammation, the reaction may be greater. Sometimes drug reactions to im injections can cause so much swelling that some of the muscle dies, which can open the door to clostridial disease that may be lurking inside the animal but that is rare and I don't think I've heard of it from sheep vaccine.
I also was under the impression the sterimatic is designed to reduce the need to raise a skin tent, so I suspect SitN is right although I doubt it makes a huge difference clinically to the effect of the vaccine. However, if anyone is ever not sure, it is worth asking the technical support of the manufacturers, in this case of both sterimatic and the vaccine.
Hope that is clearer than I think!