will sps tollerate mg at 1600?
In my experience, yes.
They can even thrive in spite of it (not because of it, for sure, but in spite of it).
As an illustration, I had a BUNCH of montipora colonies that were dying before the Tech M treatment. They were very slowly receding and dying, not unlike how a chronically low calcium level would do. But all my water parameters seemed to be in check.
I gave up on them and said the famous old line from Rocky, "If he dies, he dies".
I went on with the magnesium treatment to kill bryopsis. I raised my mag, SLOWLY, as high as 2500, if I recall correctly. It may have been closer to 3,000ppm. The mag didn't work, because I didn't use Tech M.
So I did some water changes, used Tech M, and ended up with a final magnesium of around 2300ppm, and it stayed there for a couple months.
Bryopsis died. (it came back a month later, unfortunately)
Montipora (and other SPS) not affected.
Then, suddenly, the montiporas made a comeback and are all growing healthy. The high magnesium levels had, I think, NOTHING to do with it. I think it's due to me keeping a stable alkalinity and also dosing Vitamin C for phosphate reduction. But the high mag didn't slow them down any.
Right now my mag is at 1500 and everything is happy as it can be.
Things that survived (with no negative effects from nearly 3,000ppm magnesium except one torch coral had a polyp bailout):
Bird's nest
2 varieties of acro millepora
Green slimer (acro yongei sp?)
red and purple monti cap
orange and brown monti digitata
cactus pavona
Several frogspawn, torch, and hammer species, with one torch head having a polyp bailout).
Duncans
GBTA
Nearly every common soft coral you can imagine (different tank, same system).