I do use a T2 and have done for many years,i am basing my personal opinion on what i think you are trying to achieve etc,for the most part the stock 11'' coil will be the best coil for your purpose of coin shooting at maximum depth and for a everyday machine its up with the big more expensive machines depth wise,but then we move onto the 2nd part of your question which is using the 15'' coil for additional depth on coin size item but reject small iron nails.That i am afraid dont work with a big coil,yes they give you additional depth and that is of course subject to the size of the object,if its a very large artifact size items then of course it will go down much deeper by how much ?? well how long is a piece of string.
The trouble is most folks think that buying a larger coil in the case of the 15'' coil its about the limit n a VLF machine anyway will still find you small coins and small artifacts at great depths,alas thats not the case,the big plus advantages of using a large coil is for larger artifacts and also the extra ground coverage that these coils give,but not only do you loose out on sensitivity on small targets you also can and will get problems of 'nulling' ie when you have say 2 targets under the larger coil at the same time and one is a decent target and right next door to it a rusty nail,and what the machine does it will trigger the discrimination as it sees the rusty nail as a junk item but also it would/will reject the decent target as well.
So yes large coils do have advantages and some dis-advantages as well,for trashy sites like tots lots/play ground then the stock coil or even smaller is the best option,on clear open farmland sites that are pretty clean from junk and you are after CW type relics then the larger 15'' comes into play,but then its the skill of the operator on which is the best coil for the job as alas a detector cannot make that decision for you,hence this is the main reason its advisable to have a selection of possibly 3 coils with a detector ie small coil like the 5'' one for the T2 for trashy sites,stock coil for everyday use and the larger 15'' coil for deeper clean sites but for ground coverage and also larger targets.
The large 15'' coil on paper sound brilliant and it will give you additional depth but mainly on much larger targets than coin size items,but its a brute too swing for long periods of time and not many folks that i am aware of use them,i do use larger coils from other brand names but mainly for open pasture sites or deep ploughed and rolled fields but never ever on trashy sites..
As usual these are just my personal opinion on how i use the T2,others may have different opinions.
