I'd say no on Thrawn, because upgrade characters are kind of special characters in their own right, they just don't have IC status.

To use the Stormravens to deliver Strike squads, that are better at shooting, will be to disregard half of their purpose, the idea is that your GKT's are across the board, in relative safety, chopping up the enemy while the equivalent of a flying land raider is in Twin linked multimelta range and murdering everything it can see.

If a squads better at shooting, well, there's other units that will be able to carry it better, like a Rhino with its fire points, or a Razorback, both of which are massively cheaper.

The big advantages to using a Stormraven as a transport over Rhino's and Razorbacks are it's outright speed, which confers advantage # 2, namely a cover save, while still being able to shoot a big, nasty, useful gun. It's also an assault transport with no fire points. There really is no reason to use it to carry a shooty squad when you can use it to carry an assaulting unit and have it inside a baseline camping group of Tau inside of Turn 2.

For your Strike squads, there's better, less expensive options although I do believe you're making the right decision in taking 2. I find Stormravens are a big fire magnet and taking multiples increases your chances of getting a good unit where you want it.