Firstly - glad you are keeping the motor
I have little to no experience with engines e.t.c. but I can tell you that without AVCS my car felt totally different than with it.. After new neutral position switch was fitted, the pull after 5-6k was much more noticeable.
I hope you find a good solution - Im sure Rich had an STi lump sitting in his workshop when mine was being mapped? maybe ask if he knows of one for sale?
Also Richard at East Kent Subaru Spares often has complete STi lumps for sale as my mechanic has 2-3 sitting in his workshop that he has bought off him.
Ryan