I still use my original 2006/7 T2 and cannot see any advantages in upgrading it either especially for my detecting permissions,as most of them are well away from EMI problems because most of them are deep pasture or ploughed and rolled then i rarely have EMI issues,only time is when i go under the very high voltage power lines,which does not warrant any alteration to the T2 ie shielding etc.
For pure depth then i use my favourite deep coil for the T2 my SEF 15x12 coil,not only is it light for its size,pinpointing is also good but one thing that is rare when using a much larger than stock coil its not only deep on large targets but still also sensitive on small silver hammered coins into the bargain.Of course i dont use the SEF coil all the time as you can get problems of 'nulling' especially on busy trashy sites,then of course a smaller coil comes into play predominately the NEL Sharpshooter or even the 5'' coil,the 5'' coil still amazes me the depth that it gives for such a small coil.
Wont ever part with my old favourite T2 as its found me more gold coinage than any other of my machines,of course what works for me and my site conditions may not work for other,depth can also vary from site to site,of course a larger coil will give you additional depth over the stock coil the same with any machine,but its upto the skill set of the operator when and when not to use the various coils.Of course using a larger coil does have some advantages but also can have some dis-advantages as well.