4: I still pop off IM after my second rail shot, so it's going during my cooldowns.

Yeah, but that means 1 less attack that you can get in during your 15 second super burst, or in other wiords 10% of the buffed attacks lost. If you go IM first (before hitting your cooldowns) you can have it ticking throughout the burst phase and still use all 10 GCDs for instant damage attacks