I know I don't have a 1M, but I have the full catback Berk Sport exhaust on my car and I absolutely love it. Berk is a really great company and I stand behind their products 100%! Contact Bryan through the forum (user name BerkTechnology) and I'm sure he'll give you guys great prices!
And the cats in the midpipes are NOT required to stay emissions legal in any state, as long as you have the stock downpipes, at least on the 135i. Unless the engine tuning in the 1M produces a whole lot more smog, which I doubt, I don't think anyone would have an issue. Electronically, there is no issue, as the sensors are only in the downpipe cats and not on the midpipes. They're completely bolt on, so very quick to change out. Less than an hour. A friend and I did this in about 30 minutes, and that was taking our time.
Robertm, for some reason the sound on that clip isn't working for me. It's working for every other youtube video though. Maybe it's just my computer? I'm curious to hear what the midpipes sound like on the 1M..
Here's a video of my car with the pipes. Same stock downpipes as 1M, plus the Berk sport muffler instead of the quad M exhaust. Only other mods at the time were a JB+ and an intake, so I'm thinking the sound should be pretty similar: