My touch-hole liner is recessed, although really cone shaped on the inside, but is flat with a slot for the driver on the pan side. The flinter I keep comparing it with is my old T/C that I gave to my son. It was fitted (I know this to be correct, because I found one of my factory replacements this weekend and took a long, hard look) with a liner that was deeply coned on the inside and had a large recess on the pan side for the allen wrence to fit in. The amount of metal in the center is not very thick at all. Maybe counter-boring the outside will help. Maybe I just need to do some shopping for some replacement liners for Italian threads.
Adding a slight cone to the outside will definitely help. Some have suggested also only coning the underside of the hole, leaving sort of a "roof" to help direct fire into the hole, though I have never tried this, it seems that it would work as well.
You can order replacement liners for italian threads (Lyman/Investarm) from Track of the Wolf. They have them in both Ampco bronze and stainless steel.