I wouldn't say "double" anything. There are style guidelines for Imperial IPAs and Russian Imperial Stouts, but not much else. I've had IIPAs anywhere from 7.5%-11% and 70-100ibus. Its really up to you and your definition of "imperial". Traditionally that means more ibus and higher abv, but not necessarily "double" the amount.
I would look for commercial examples of what you're looking for and try to find a clone recipe.