Don't know about 2.7's, but both my 2 2's sound better on the 8 ohm than the 4 ohm. Why, don't know given the impedance curve, although it does rise in the midrange/treble a bit.
Thiel owners please chime in.
Just switched from 4 ohm to 8 ohm with my 2.7s and it's like WOW wouldn't have thunk it. They aren't the most efficient speakers obviously but my BAT VK55 sounds so much better now. What has everyone elses experience been?