To the side of our house I added a bedroom and a bathroom. [ approx. 336 sq ft ] New area has an almost flat roof attached under the gutter of old house. The new floor is 27 inches lower than old floor. To get heat and air to new room I came off the end off trunk line in attic with a 90 dropped 27 inches and turned horizantal with another 90 , piped into the new very small space above ceiling. I then placed an 8 inch tee, reduced to 2 six inch runs to vents.
It`s always colder in the new room. [ thermostat at 74 with outside temp at 48 , new room is 66 to 68 ]
Question: what can I expect if I place a 8 inch duct fan in 27 inch drop ?

Probably not much. The trunk line is already taxed to the max. Just add a small electric heater to that room and save some grief. They are not real expensive and you can put that heater on its own thermostat.

Option B is to be real cheap and just buy a small box heater and plug it into the wall.

When you added another duct to the end of the trunkline I would guess you took away static pressue from the other vents feeding from that end of the trunkline,, this will hurt the other rooms, we never put a duct on the very end of a trunkline