Well, to answer all the questions.... The SiR seats aren't really any different from USDM Si seats, except the JDM driver side (USDM Passanger side) has the back support.
This might not hold true for the 90-91's, but my 89 DX had different seat rails then all the other 89 trim levels.
As for that, I think you might be right... Unfortunately. As far as botching both sets of seats, i'm just going to sell them... Really unfortunate, but rather than destroy them, i'd rather sell them in mint condition to someone who wants them more than me. Unless someone comes through with a really badass idea......
FS Link:
FS: EF9 SiR Front and Rear Seats
-JSpecMugen