Shure is generally known for their quality, so that part of it is fine. As far as it being a USB mic, I've never used one so I can't really make a meaningful comparison against a microphone in its traditional setup with a quality preamp and converters along the series.
I can say, however, that in addition to quality microphones, quality preamps and converters are also where you get your best sound, and recording studios are definitely using the best of the best in most cases. That is to say that they are NOT using USB microphones.
We can draw from this a conclusion that the best possible scenario will not be reached with the microphone you're planning on buying, so we then ask ourselves if we're willing to put the bar lower, and if so, by how much?
Well, considering that you're probably somewhat new to recording, your weakness won't really lie in the fact that you don't have top-notch gear, as the quality of the gear is not going to make a speck of difference if it's not utilized properly in both the recording stage and the mixing stage.
With that considered, the quality bar should be set approximate to the level of your skills and experience, but with a little room to grow into. In this case, I would say that such a purchase as the one you are planning on buying is good for that.
Whether you will be able to produce music that is on par with material that is sold on iTunes is another matter entirely, and is not dependent (solely) on what type of mic you buy. Like you said, it's about the mix, but it's also about the environment in which the material is recorded, the techniques employed in the recording process, the quality of the equipment at all points in the signal chain, and your experience level regarding all this in general.
I say all this to say, that your primary concern should not be one particular thing, but an aggregate of the entire set of factors that play a part in professional recordings. If you accept and understand this, you will be less likely to be disappointed when one single solution doesn't address the larger problems you may encounter.
My recommendation would be to go ahead and buy the mic, since it does not require additional equipment that traditional microphones require, and focus on getting the best possible result with that. The more you learn about all aspects of recording and mixing, and the more you implement what you've learned, the better your overall results will sound. When you reach a point where you cannot possibly squeeze another drop of quality out of what you have, THEN it's time to start thinking about upgrading your limiting factors.
This is a good strategy because it forces you to learn how to get the best possible sound based on knowledge, which then can be translated to better gear. It doesn't work the other way around.
With that, good luck.
And remember to utilize knowledge resources wherever you can find them, and be patient. If you expect to be making professional-sounding recordings in less than several years, you should re-evaluate your approach.
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