I take on board the comments about greater effeciencies with altitude but in the real world, the wind has usually by far the greatest influence upon performance. A 15kt wind on the surface will often be 30kts+ aloft. That's a 10% performance advantage/disadvantage for a 150kt helicopter; and there aren't too many that can achieve that. Of course, wind direction and it's associated changes with altitude is also a major factor to be considered.
Shawn
The A109A and C series often have TOT limited cruise over here in the UK winter. Generally as a result of the need to turn on the air-bleeds (anti-icing and cabin heat). A little bit off-topic but another factor to consider in performance planning in the real world.
JJ