We do have an O2 sensor bung after the cat for your O2 sensor.
Currently we do NOT include an extra bung for a wideband sensor. If wanted we could make a downpipe for you that does include the extra sensor for your tuning needs in the future.
If you are interested, give Mastro a call and let them know about the additional bung and we can manufacture to suit.
Regarding leakeage and flex; the stainless flanges we machine have holes that allow for a little manipulation so the exhaust can be properly lined up. Once tightened however the exhaust is guaranteed to stay leak free. We use a Stainless Accuseal clamp on the muffler for a leak free fit as well. Finally the downpipe is bolted to the turbo and also has a bracket that attaches to the factory mounting location. This portion will not flex and break seal. The catback once tightened, as mentioned before is rigid but is allowed to 'swing' on the factory rubber hangers.
what a mouthful!
In short, you are guaranteed to not have a problem.