I found it quite similar to the Custom in terms of EQ balance, actually. I'm sure a couple of people will disagree with me on that, but to my mind it has a touch less treble and a healthy bit more mids. I like the stock Custom in a Les Paul or a thicker bodied instrument; but in a thinner mahogany guitar like an SG the C8 kicks it's ass.
The C8 keeps the beef and thickness from the Custom but adds a lot more of an organic nature. It has more natural sounding overtones and is a lot more musical to my ears. The treble is a lot fatter and warmer than the regular Custom. The mids are fatter and the bass is a lot warmer and more complex, while barely losing any of the tightness. It keeps about the same level of output as well, so if you're worrying about it not hitting your pre-amp as hard...don't.
It's a great pickup in an SG. My second favourite was the Custom 5. The point is that it's a really easy mod to make and reverse if you don't like it, and the experimentation and learning experience alone is well worth the price of the magnet itself. You should get the Custom and a couple of magnets. I'd be extremely surprised if at least one of them didn't work out for you.
I'd also be surprised if you didn't settle on the C8.
Welcome to the forum, by the way. Stick around. :friday: