You're right about the A/F correction. It should be positive for a pre turbo leak. You might still have another post turbo leak.
The advantages of the aftermarket silicone are:
1. Easy installation
2. Seems like it would be more durable than stock which seems to have a weak point where it connects to the turbo. The stock ones seem to last 8+ years though.
I've installed 2 of the cheaper aftermarket ones on friend's cars, and the fitment was just awful. The higher quality ones might very well fit better though, but based on my experience with the cheap ones, I'd just stick to the OEM. It's not THAT hard to remove the IM.