I would say, yes, it is dangerous. You only need a small hole/leaky seal somewhere and fumes will enter the car. Have you ever driven in a station wagon with the tailgate up? If you have, you will know what I mean. Even though the car is moving forward, the fumes find a way in somehow.
That is why the make exhaust pipes go the length of the car and exit at the back. Of course for drag racing etc, you can do what you want within their rules, as it's only for a short period of time.
Youcould always replace the whole rear section with a piece of 3" exhaust pipe - I think you would find this nearly as good as running nothing.
AU1 XR8 Auto 147.9 rwkw (approx 215 fwkw)
AU2 Wagon HAD Auto
17s, Pacemakers, 3" Intake & Pod filter (The Family Hack)
FPV-XR Club ACT