Made this as one of the two cocktails for my wedding a couple weeks ago (the other was Bourbon with a Peach Ginger Shrub made by my now-brother-in-law who tends bar at a terrific place in town). Both drinks went over amazingly well and 4.5liters plus of the hard stuff (that's just the volume of the spirits, not the mixers) was gone within the first hour or so of an 80guest wedding (with wine and beer being served as well). The syrup is 1:1 with a bunch of rough chopped fresh mint stirred into the simmering honey:water mix (and then obviously strained). I also added a bunch of fresh mint to the jug in the fridge to steep further once it had cooled, and that steeped for about 12 hours). We served this over rocks, but if you could get it chilled enough and appropriately diluted, it could be delicious straight up as well.
2.25 liters gin
850ml mint honey syrup (see above)
425ml lemon juice
425ml key lime juice
Water to taste
Mix ingredients in bigass jar/bowl/jug. Add fresh mint to container if it's not minty enough. Chill. Add water to dilute depending on how cold it is, wether you will be serving on ice or not etc.