I have attached a screen shot from the 6th edition of the performance handbook showing the different undercarriage options that the D6C had for show width.  Considering it does not even list a 30” pad option for the standard gauge tractor, I wonder if a: someone put them on themselves to try and make “homemade” LGP tractor, or b: if Cat later offered it as a wider pad.
The standard gauge D6C is 74” and the standard pad width as you can see by this chart was 18”.  And had optional widths up to 24”
The LGP gauge D6C is 83” with a track pad width of 36”.  That means the track frames were spaced out an additional 4.5” on each side to allow the wider pads for better floatation. 
If you truly want to narrow your track pads I wouldn’t go any narrower than 24” personally, you never know when you might be in a situation where that little extra width gets you out of a bind.  You can see that a standard gauge D6C with the 18” pads has almost double the ground pressure as the full LGP gauge tractor.  So you do gain floatation as the pads get wider.  
I think the best option for cutting them is oxy/acetylene, just give yourself a nice straight edge to follow and cut equal amounts off each side of the pad.  Don’t just cut off one side like I’ve seen others do, otherwise you still have the same problem on the original width side of the pad.
Yes wide pads can cause premature wear in undercarriage because of the extra stress that can be added by hard uneven terrain.  What kind of work do you plan to do? How often will you be in softer conditions? Are you working in lots of big rock? These are the questions I’d consider before cutting to narrow.
The other option is you could watch for a set of narrower pads to come up for sale and swap the pads out rather than torch cutting them and potentially weakening the metallurgy of the steel from the heating and cooling process.
Hope this is of something use, 
Nathan