I agree with what everyone has said above. I actually called the SCCA about this, and they said that the safety engineering behind the stock tank is very good. The stock tank, sitting so far forward on top of the frame rail/axle beam, is very safe.
Another thing to consider is that folks often locate the fuel cell behind the frame rail/axle beam, which puts it in the crush zone for rear-end contact. This certainly negates at least some of the advantage of having a cell.
Finally, you are driving a Jetta so the trunk puts even more distance between the stock tank and the rear bumper.
If I thought it was more safe I would have a fuel cell . . . I race a Jetta, I have a stock tank, and I feel just dandy about it.
DE