Catless Downpipes
I have catless downpipes, got cheap ones, work great. MHD 2+ tune
I had the opportunity to do some emmissions testing on a few vehicles as a part of a research project, so I slotted mine into the test group. Was done at a VTNZ station & they checked the cars over. Noone noticed the lack of cats in the downpipes (they are fairly buried and the secondary cats are visible, so it needs someone knowledgable on these cars to pick it up).
Long story short, mine was a little "dirty" on its emissions when cold, but once warmed up, it was as clean as any other car tested.
If challenged, take it for a strop round the block & tell them to stick the sniffer up a tailpipe. They will give up & pass you as your car won't fail unless it has other issues. I was worried that the MHD tune might over-richen it, but no issues at all.
Vibrations
My car has a very slight vibration noise just above idle, then sweet as. one of the pipes is just very slightly contacting an unidentified bracket & this makes some noise. Get the car up on stands, start the engine & go looking. Ideally get someone in the car to vary the revs up & down until you hear the vibration & follow the noise. Welding gloves highly recommended for this exercise btw
Pops and Bangs
Nope, have none, but have had experience in a variety of cars. The most likely cause is an air leak in your exhaust, most likely the joint where the downpipe comes off the turbo. The closer to the exhaust port of the cylinder, the more likely to pop and bang. As a younger hoon, I used to deliberately make my car exhausts bang, but as a more refined gentleman I do prefer to fly under the radar a bit more