It's a tough call. Check the style guidelines to see if spices are appropriate and at what level. Example: We were doing Belgians in our BJCP training last night, a style lots of brewers want to spice. In general, the guidelines stated low levels of spices were not required but appropriate, as long as they complement the flavors of the beer. The only style I can see that mentions "Imperial Porter" is Baltic Porter, spices are not appropriate in that style. You may be thinking of 20C, Imperial Stout, but again, spices are not really appropriate to that. So 30A is about the only place I can find for it if you're going to make the spices noticeable. You may be able to enter this as a Fall or Winter Seasonal as well, check the description and see if your beer fits.