Couple of things to consider. Is it finished fermenting? If it is finished (same gravity reading over several days) then add sugar at bottling. If it is not finished, I would recommend waiting for it to finish, then add sugar at bottling.
Another thing, what is the estimated attenuation of the primary yeast vs. the T58 bottling yeast. If T58 attenuates more than the primary yeast then it will take the gravity lower in the bottles and you need to account for that when calculating the amount of priming sugar to add.
If it is not finished, and you want to bottle it, it is really a guessing game to decide whether you need sugar or not. Too many variables to guess at, unless you have a lot of experience with that recipe, those particular yeasts, and their relative ability to attenuate.
Also, the priming sugar should not impart any residual sweetness. It should all ferment, unless the yeast is pooped out, but my experience with T58 is that it will be fine in a 1.080 beer and will ferment all the priming sugar.