Does BiS take into consideration the Creation Catalyst, or will do so in the future once you have more time to develop it?
Obviously for the tier-bonus pieces that are at the highest ilvl (Shoulders and Chest) for a certain difficulty, they are probably already the best, but for the ones that are not (Helm, Legs and Hands), there might be options in the raid from the last three bosses that could be converted into tier pieces to give a better ilvl. This is only considering items from raiding, there would also be numerous options from mythic+ and pvp, etc
Additionally, if you had the full list of bosses from one difficulty, and then a few bosses from the next difficulty, based on your current progression and near-future progression, there might be better ilvl options from the first few bosses at the higher difficulty level.
Also, for the slots that don’t get the tier bonus, the change in secondary stats may still be an upgrade over the original piece.
Thinking of a way you could implement this, rather than having to include an extra item for every single piece of loot that you look at for the BiS optimisation, you could figure out the maximum ilvl that could be converted for each slot, and just add a single tier piece at that ilvl to your BiS candidate list. You could leave it to the user to figure out which item/s they need to convert to that ilvl, or you could put in a list of items that would work for the maximum ilvl.