Per the original question, the pressure in the insert would be just like in a typical chamber for the gage shell. Peak pressure occurs typically while the payload is in the forcing cone or just clearing the cone (depends on cone length and powder burn rate). As the payload clears the adaptor, it will see the barrel as an expansion. Assuming the wad obtrudes sufficiently to seal all gas, the pressure as the wad clears the adaptor will be "typical" for the shell in a normal chamber. As the wad travels further into the barrel, the volume behind the wad will increase faster than typical and the pressure will drop faster than normal. The pressure drop will be off-set by the increased area of the obtruded wad's base. The force accelerating the payload will be about the same as normal. Therefore, velocity will be about normal.
The kicker here will be the first inch or so of barrel travel. A high pressure shell will still have fairly high pressure at this point. It would be necessary to calculate barrel stress at this point to say for sure as to safety. Note also that firing high pressure shells in an older gun can give too much back thrust on the breech face (compare peak pressure times shell area for the gun's chambering and for the proposed sub-gage shell. Post back if this is not clear or anyone wantws to discuss further.
