I also shot at ISO 12. Have heard that you can scan as colour and then inverse. Has anyone tried that?
IR film is always quite grainy because the grain is always quite large as it has such a large sensitivity due to it being extended into the near IR spectrum. It may be shot at ISO 12 like Joxby says, but remember that thats with the filter factor so its actually 200/400 speed film (depends on IR film type).
This is strangely exactly the opposite of my experience with the Rollei IR film. I found it be almost grain free last summer when I shot 2 or 3 rolls, and developed in Rodinal. The only example I can find I've got online is this one: