Sounds like you already know what you have to do, except that you don't want to drop Streamline, you want to drop Fleeting. A max Streamline and the Essence helmet give you 55% efficiency, which is only 20% from max and has no offsetting negative.
For duration, I run max Continuity and Narrow Minded at rank 7 (minus 48% range) and max Overextended to offset Narrow's range penalty. I've never timed it, but it feels like I'm invisible for 30-40 seconds at a time. Constitution is overkill and only 28% anyway.
So 4 mods (Streamline, Narrow, Continuity, Overextended) and one ugly helmet will give you all the efficiency, duration and range you could possibly need. Use the other two slots for whatever defense you want to set up. I use Rage & Quick Thinking.
Incidentally, Loki is the only frame that doesn't require a single forma to optimize. He's just an amazing frame. Wish I'd discovered him sooner. I even have a point left over to upgrade Narrow Minded to 8, but I feel like it's pretty good at 7. IMO, that ugly Essence helmet is as necessary for Loki as the Vanguard helmet is for Rhino. Keep an eye out for alerts or spend the plat.