As BillieBob states, the SFI qualification is based upon completing a TRI course. With my operator an SFI can add the TRI endorsement to their licence after completing a prerequisit number of hours instruction in the sim. I also believe they must have Captain status and possibly a stipulated number of hours as PIC. There is a requirement to carry out simulator training sessions in order to maintain recency. However I believe this to be minimal.
Unfortunately, in terms of remuneration, your status as TRI within my airline holds little weight. However it is a useful thing to have on your ATPL, plus it seems to put you towards the top of the pile for future training positions such as LTC and TRE.
Just to add, the vast majority of SFIs here are FOs.