Assumed temperatures are calculated to satisfy the regulatory requirements of 2.4% (for a twin) in the second segment (gear-up to the commencement of flap retraction).
If you are doing 170Kts Groundspeed you should see at least 400'/min on the VSI, if not then obstacle clearance may be compromised.
The calculations in the OPT are all well and good but on the day, the wind may not be as forecast, the temperature may be warmer, the aircraft may be heavier, there may be an inversion, etc and the aircraft doesn't performance quite as expected.
When the performance isn't adequate, consider TOGA thrust.
I don't fly the 787, but have 17 years+ on Boeing twins. The B777-300 at MTOW is 'interesting' ;-)