The diference keyer isn't the most powerful thing in the world, but if it's getting you close, more power to you. As far as spill suppression eating into your key, the only thing I can think of is you have the spill suppression operator on before the keyer. Make sure the SS op is after the keyer so it effects the keyed image after the keying has been done. If you are doing that, I'm not sure, like I said, I don't really trust the diff keyer unless a shot was set up to be difference keyed when shot.
I'd try the Discreet Keyer. Especially if your using a green/blue screen. Or if you have it, try AdvantEdge, it's an excellent keyer as well.
Edge blending won't always fix the problem, sometimes it even amplifies it. There are so many factors to getting a good edge - what format are you working in, is it lit dark or bright etc. Something that sometimes help is grain removal. You can use the Grain Manager op to smoothly remove noise from a clip so you can get a clean edge. Just don't forget to re-introduce it.
There's also plug ins called stuff like Edge Flash or Edge Blend in different packeges that will help you mix edges nicely.
I could write a pretty big chapter on keying, but in a nutshell, tweak away. Sometimes you'll need to isolate areas with Selection and use a different keyer for the selected region, sometimes you'll need a bit of roto or paint.
Hope this helps,
jon a