G'day
I bought and used the NF 12" evolution coil on my early model 4500 and as I was not overly impressed with it, my mate also used one on his late model 4500 and he was not fussed with it either, but he does really like the 15" evo coil which he finds runs more quietly, its all about sensitivity and sometimes depending on the ground type we have found that an overly sensitive coil can be problematic, the benefits of the coil having greater sensitivity can also be its downfall in some ground conditions and it can tend to become erratic and have sporadic falsing making it harder to use, much like using a minelab 8" mono around banded ironstone areas.
I have found the minelab commander 11" mono better in these conditions as it is not overly sensitive but is a far better coil to use on hot iffy ground where there is a problem with hot spots and variable conditions, running more quietly is the key to getting deeper gold as the target signals can be vague and very easily missed if the coil you are using is noisy and unstable, they will also go very deep for the size as it has proven time and time again, a lot of people don't use them in favor of something else maybe this is only because it comes as stock standard with the Gpx machines, but in my opinion and experience it is a very much underated coil.
I once saw on a interview with Bruce Candy by Jonathan Porter, Jonathan asked him what the optimum coil size was for the pi machines, not sure what year or what model pi machines he was referring to but his reply was a 15" round coil?, I found that interesting because minelab do every size and shape coil but not a 15" round coil in either mono or dd version?, with this in mind I was thinking that the 15" evolution round mono coil could very well be the best size to use on pi machine but more particularly on the Gpx models, I have not used one on my 4500 but have been tossing the notion of getting one about for awhile, but also still would consider the 18" round as well as I have been hearing that they are also a very good coil too, all you can do is use the "suck and see" method and buy one and try it for yourself, its the best way to find out what will best suit you and what works the best in the ground conditions that you normally detect in.
cheers
stayyerAU