The problem is that the glyph isn't -20% to TPS, it's -20% to total threat.
IE: You've been doing 7k TPS for 1 minute. Pop HoSalv. Lose roughly 84k threat. You're effectively running at -2.5k TPS for 10 seconds.
Pop it at 5 minutes? Boom... lose over 400k threat.
I imagine a /cancelaura Hand of Salvation macro could work to minimize the threat loss, assuming that the reason for glyphing this was to save your rear with a Sartharion breath-esque attack.
It will be interesting to see how the usual threat lead we enjoy thins out, as DPS itemization gets better. I think threre was a blue who commented on how while at Naxx levels tank threat is pants-on-head retardedly OP, maintaining such a secure threat lead will become more challenging in latter WotLK.
