I did some digging around on FF Logs and saw that Guren typically averages about 3% of the total DPS output for a character. If you take 7k dps as the total (totally fine for 8 main raid scenarios with buffs), then......

Guren DPS Contribution Estimated 3.2%
7000*.032 = 224 DPS

Guren is 4/3 stronger than 2 shintens, so.....
224*.25 = 56 DPS.

So my guess from my earlier post was a bit too high. My estimated DPS loss from using 2 shintens instead of guren is only about 50 DPS. That tiny margin only matters to the absolute top of the player base, not the rest of us scrubs lol