Just curious if anyone knows off hand.

Does the P220 combat already come with the appropriate slide release mechanism for clearance on the grip?

The release should clear any aftermarket grip.

No apparently the Sig upgrade kit is slightly too bulky to clear, some Sig models are factory with the new one that clears I would guess anything else.

First off, what is this "upgrade kit" you are referring to? Got a picture or part number?

Second, what's different on the P220 Combat Slide Catch Lever (SCL)? I'm not aware of any difference...

To add:
On some grips you have to maneuver the left side grip panel under the SCL and de-cocker, it doesn't just slide under them.

Are you referring to the E2 grip?
https://www.youtube.com/watch?v=VYfuFAzn4tc

If it's an E2, I personally don't like the size of the E2 if that's what you're referring to as an "upgrade", but to each his/her own.

I'm curious about this as well. The slide release on my Combat has fit with no problems with any aftermarket grip I have used - including Hogue rubber grips (fat!). Couldnt tell you on the E2's, I dont like them. The only thing I see that comes different with the E2's is a new decocking lever.