But you do?
No, but pretty much all Japanese people find it offensive, so I'd say it's fair to say that it's offensive to its target group.
Listen, I don't really care what words people say on the internet. The beauty of the internet comes from the freedom to say things uninhibited. However, you are not equipped nor qualified to decide whether 'jap' is offensive. If you really wanna say it, just do it and don't try and justify it.