Hi Alan,

What the "Goat Link" does is drop the derailleur down lower so that it can clear the larger rear cogs. You can use the "Goat Link" up to 40T with dual chain rings but not the 42T "Goat Link" at least according to the mfg.

I'm running a Shimano Ultegra Di2 6800 GS "road" derailleur which has a mfg spec of 32T largest cog. If you max or reverse the "B" screw you can get it to clear a 36T. What the "Goat link" does is drop the dreailleur down approximate 15mm lower so that it can clear the 40T that I'm running now.
In your case the "Goat Link" would do the same thing. Assuming that your road derailleur has a max cog of 32T it will allow you to run a 40T. With your existing shifters and derailleurs.

You're right what the "Shift Mate" does is change the pull ratio on your shifters so you can run a mountain bike derailleur with a road bike indexed shifter.

I'm putting this part in for people that don't understand not sure if you do or not but here it is.
This gets confusing so hopefully I can be clear on this part if not let me know and I'll try again.

The cog spacing for 11 speed cassettes is the same for both road and mountain bike cassettes. I'm talk Shimano can't comment on other brands.
So what this means is that the derailleur has to move the same amount to shift between cogs. The reason for products like the "Shift Mate" isn't because the cogs are spaced different but because the shifters used on mountain bike derailleur and road derailleur have different pull ratios because of the lever travel distance. Think brifter on road bikes and thumb trigger on mountain. Your road bike brifter has approximate 4 inches of lever travel, I didn't measure it so I'm going of memory. The thumb lever on a trigger shifter has approximate 2. So this means that the trigger has to pull more cable in it's two inches than a road does in it's 4. So if you just hooked a road brifter to a mountain bike derailleur you'd end up changing two gears instead of one. (I'm just trying to give an example here so it might not work exactly like this put hopefully it conveys the problem.) The "shift mate" corrects for this indifference.

So onto the next step
So your option are either mountain derailleur/shift mate, or Goat Link and each has it advantages.

The advantage of a mountain derailleur is that the derailleur will have higher ground clears in relation to max cog size. The other advantage on the higher end derailleurs they have clutches which help stop chain bounce. Which is a real issue on mountain bikes but not so much on touring since you're not as likely to be doing jumps on a fully load touring bike.
The advantage of a Goat Link is you can use your existing derailleur and shifters. You just get to use a larger cog. The disadvantage is you don't get a clutches derailleur. On the other hand if you want or need to you can revert back to normal road gearing by removing the Goat Link and putting on a standard cassette.

So I'm running a full "Road" group Ultegra Di2 system on my bike, because I'm running the electronic shifting I can't just swap out the rear derailleur to a mountain. The electronic system shuts off if you mix road/mountain derailleurs. I could run a second battery and junctions to run two separate system one for front and one for rear. I chose to go the cheaper route of the Goat Link.

Hope this makes sense
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