So Sahir's can extend their lifespan in a couple ways. First, they can reduce their age, which allows them to keep their penalty to a minimum. Secondly, they can improve their arts, further lowering their penalty, and in fact giving them a large negative bonus. Finally, they can remove aging points so the only thing that matters is a crisis.
Now the book says that a Sahir will eventually die as decrepitude builds up, since they can't remove it. Not true. Sure, (most) can't remove decrepitude or freeze the aging process wholesale unless they win the summons lottery, but that doesn't mean they will die. Assuming they spend a constant fraction of their time learning their arts, specifically alchemy and medicine, the chance that they will get crisis exponentially decays over time. Which means the total chance that they get enough decrepitude to die converges to a number less than one. So barring a violent death, or slacking in their studies some Sahir's will live forever.
Next they have warping. This gets even more out of hand. With enough confidence, the Travel Art, or Faith they can ensure its a good experience. So they come back with a higher confidence score, more virtues or a higher social ability.
What this means for war with the Order: The oldest Sahir's are 400ish, have obscene casting totals, can bind the most powerful of jinn. You really better hope one of them didn't found a mystery cult, because they'll be able to initiate themselves, and everyone else with... anything they feel like. Their best attack spells don't actually need to penetrate magic resistance being a natural change. Just a finesse roll which is rather trivial when you have a bonus in the dozens. They will go first in every fight. Oh and they might even have magic resistance courtesy of the Infernal. Speaking of the Infernal, you can't detect their soldiers. Period. I hope a demon didn't sneak in to your sanctum and murder you while you sleep that would be just awful. Is that friendly Redcap a demon? Is there a demon looking over your shoulder while you study Parma? The worst part for the Order? Sahirs can get into the Magic Realm and strike at vestiges. People or covenants, or whatever can be destroyed with no discernible cause.
So if the Order actually goes to war with the Sahir? Underestimating them would be a very painful mistake. If the Sahir go to war with the Order instead? The Order first needs to find out who is behind the demon attacks. Also that there are demon attacks, I hope they didn't try to rely on magical investigation. Either way you can make this an utter horror story for your players.