The genuine UE silicon has always worked best for me with wide nozzle tubes. I say genuine because like anything, it can be counterfeited and like most counterfeits, it is never the same as the real deal. Like most ear canals, they are asymmetrical. Some more than others. I think mine are on the more side. Below are my Atrio custom sleeves to exemplify this, and notice the big difference between the left and right (red tip). Off the topic, I do feel the custom sleeves are a good investment for the mg7. It keeps the signature low bass, provides unparalleled isolation and comfort, preserves the neutral sound, but brings out more of the mids due to the sound tube being closer to your ear drum:
I've owned all the super.fi and tripe.fi universal models and always found the stock UE tips the best. They are also good to substitute on other manufacturers like Sennheiser and Grado IEM's. What I really like about the UE tips is their stiffness. Similar tips don't have enough stiffness when formed inside the ear and gives away to easily, along with that goes the seal. Below is a side by side comparison of the stock large ASG-1 silicon tip and the UE size large. You can see they are very similar to shape and size:
The ASG-1 tip is on the left
Where the two differs greatly is in the side wall stiffness and this is what makes or breaks the seal for me. I'm applying about the same pressure as I'm holding each tip and you can see how the ASG-1 tip easily collapses:
Regarding the sound difference. I didn't really notice any significant sound changes and didn't really expect much change from the beginning. The biggest change is the fit and seal. For me, it's like night and day, and makes a world of a difference.