When the OP asked a question about the performance characteristics of the Record Monza, why are folks going out of their way to steer the conversation into considering alternatives that they have a personal bias towards? The question was specifically aimed at performance characteristics of the RM.
If the OP decides the RM isn't going to be their choice, there are lots of threads on here that speak to the alternatives, and what owners like about them. Although I personally own a RM, have had zero performance issues with it, and do personally like the way it behaves.... my hyping of the product doesn't answer her question. And discussing another exhaust that doesn't provide the same functionality doesn't help either. The Goodwin version is the equivalent of a RM with the valve welded in fixed in place. It's a fixed solution with no ability to generate a variable tone that doesn't involve mechanical additions or subtractions to the system. Nothing wrong with that at all, but they're not the same thing. Apples and oranges.
People have said the Goodwin is quieter than the RM, but if you have an Abarth model, the stock exhaust is also tamer than the RM.... but for those with the Lusso or Classica, then the RM vs Goodwin is a valid conversation as they're a different stock exhaust configuration. But, the OP is from an Abarth owner, so....
I think Greg's answer comes closest to the question being asked. The valve is never fully closed, and some exhaust gases always flow through all four pipes. Pressure will push the valve open further, and the RPM at which that occurs will vary. It's fair to say that some people have has problems with the flap seizing in one position or another, but that's because it's a moving mechanical component that requires maintenance. If you never maintain your fluid levels, tire pressures, brake pad and tire wear, you'll have issues with those as well. A dab of anitseize compound is the maintenance that's needed, and if used, you won't have an issue with the system.
When first started in colder temperatures, my vehicle will initially idle around 1200 RPM and the RM is louder. After about 30 seconds, the engine begins to warm up and the idle starts to climb down to around 750, and the exhaust note tones down along with it. As long as I'm not applying agressive throttle, it remains relatively tame.... better tone that the stock exhaust on the Abarth, but not horribly loud. But, if I agressively mash the throttle, it opens up and growls like a wounded beast, and that's the sound I enjoy hearing come out of it. At regular cruising speed on the highway, it's back running in it's quieter mode, and the RPMs would be in the range of 3000 to 4500. If I downshift to 5th or 4th and apply throttle to pass a line of vehicles, the RM will open up and make itself known again. In reflection, I think it comes into play relative to when the turbo is generating boost, as opposed to a specfic RPM band. If you run the Abarth in Sport Mode, the boost comes on lower in the RPM range, and the RM also becomes more agressive as a result.
So to give the Reader's Digest condensed version, iIf I do a gradual acceleration as I work through the gears, it will remain in the quieter state. If I come onto the throttle hard and the turbo spools up, it will be loud. It's more to do with boost pressure than RPMs.