The whole point of introducing combat length variation was to simulate a random tweak on the combat length. Not to simulate the difference between a 3m fight and a 7m fight. So something like 6m +/- 30 seconds would be fine, any variation that is +/- 2 minutes is not realistic.
To get the 5:45 value I tested with 0 variation in length with switching glyphs.
Personally I generally sim 7 minutes +/- 15% giving about a minute in variation either way for my own BiS research purposes. In sims for EJ BiS comparison, I go with the OP times.
Otherwise, I may use tighter times per fight I have logs for if I wish to know particulars to when making myself slightly depressed and want to do better in future encounters.
As of now, the Professor is the only progression raid encounter that I would consider using FE glyph (fight length is too short on the other bosses). I do not actively PTR so I cannot say if future ICC bosses may benefit from this glyph choice; however, I do have a few different glyphs in my bags should I feel the need to switch.
For maximum theoretical DPS of the BiS with a 7 minute =/- 15%, FE glyph will win. +/- 35% FE glyph still wins, but by a smaller margin.
So I digress after getting OT. Looking realistically with the moving and raid awareness, FE glyph isn't practical. Fights that are static for DPS are less than 5 minutes and fights that are longer have lots of movement thus far.
Edit: Also, thanks Ryethe for the logic behind the wide length variation.