Cannot comment about the Platypus coil,but i use Coiltek 14x9 Mono,Commander 15x12 mono,Detech 18'' Mono and the badboy 20'' Sierra Grande Mono all give me the depth that i require,of course what works for me may not work for others.The downside of using a large coil is although you gain the depth you then start loosing sensitivity too small individual coins.
Another bonus of using the Minelab mono coils i can also use them on my TDI Pro which does offer the high/low conductivity discrimination,unlike the GPX disc which is only good basically for the top 4-6'' and only with a DD coil,the TDI Pro setup is more effective at much greater depth rather than just say upto 6''.