I've been doing a lot of digging regarding the Cobb/jb4 question with our is'.
One important factor to consider is if you have DCT vs. 6mt
If you have a manual, get a jb4 and call it a day. You may eventually need a back end flash as well depending on how deep in the tuning pool you swim.
If you have DCT, it is highly advisable to start with a flash, like Cobb. There can be potential problems because a piggy tricks the dme into doing things it wants. Unfortunately, this also fools information that helps control the DCT. Line pressure for example. This can cause slippage and wear and tear on your expensive transmission.
A flash tune like Cobb sends accurate information to the DCT so it can behave accordingly.
Ultimately however, there are lots of cool features on the jb4 that you can't necessarily get on a flash tune. So, get both! Bms has special maps for the iS that can be installed with a Cobb (or oft for that matter) that support the jb4 and will be much better for your DCT.
I started with a Cobb. Mainly because I am still under warranty + maintenance package so I have frequent trips to the dealer ahead of me. So it's nice to not have to pull the jb4 every time.
But I have decided to stack it with the jb4 eventually, probably after my next oil change and brake service.
That's my .02