To answer your second question, I do not know and I would really like to see the 5" coil offered by Kellyco and some of the other larger dealers.
As for your first question, I picked up the coil about 2 months ago and have used it several times. Short answer, yes it is very good in trashy areas and in wooded areas with a lot of undergrowth. I often hunt for Civil War relics and, recently, even pre-Civil War sites. Many sites are in wooded areas and, except for in the Winter, these areas are full of growth that "reaches up and grabs the coil". The 5" works very well even though I don't get the coverage of the DD coil. The 5" really capitalizes on the excellent separation between trash and good targets that the T2 is well known for. I have uncovered several targets that were next to trash and I was getting good and bad tones in my headphones at the same time. I usually run on 2+ tones so when I got the good with the bad I dug and found a good target along with a piece of iron or trash most of the time.
Is the coil worth getting? It really depends on where you are hunting. The DD coil that comes with the T2 is very good in trashy areas where you can get a decent swing on the detector. I searched an old rail yard recently for coins. Needless to say the place was "trash central", but there was nothing to interfere with a good sweep motion of the detector. I used both coils and did not find any advantage in separation with the 5" so, after a while, I stuck with the DD.
if you have the conditions I think the 5" is worth having, but you are the best determinant of that.
Good luck!