I have never owned the Max 15'' coil but i hve seen and used one on a friends MXT and yes it is massive,it of course needs alot of thought before buying such a coil as often the negatives far outway the plus's,if you solely plan on using it for deep work and nothing else then it could be a good move but if the plan is using it for long periods of time a harness could be seen as a good move.
Will it give you depth over the stock coil,the answer would be yes,by how much not as much as what you would think it would,a bigger coil real advantage is of course finding larger targets at depth but you will loose out possibly on smaller targets,so only you can way the advantages up.
One of my machines that i shoot from the hip but on specific sites when searching for hoards is the DFX but i also use a larger heavier 18'' Detech coil,this is even heavier than the 15'' Max coil,and to get the maximum use out of this combination i have found that i needed to run it in 3Khz to make it owrk,but once again its only really good on large items,but this is is what i am looking for and not single coins,it even mentions this on the coil box that you would be wasting your time really looking for coin size object.
But if you have a specif use and plan on using it alot then buying such a coil could be seen as a wise move,but if you plan on searching in tots lots for small change then it will be a total waste of time and money.