If you put in an assumed temp of 60C is that at sea level on 10,000ft runway or at altitude, hot humid day, on a 6000 foot runway?
An assumed temp of 60C is an assumed temp of 60C. That's the point....
Of course, on a short runway or with a hotter OAT your V speeds will change and thus you will need a different amount of thrust to meet the various requirement, which will affect the assumed temperature.
The Airbus FMGC does not calculate any speeds IRL - in the Aerosoft version if you put in a flap setting it will give you a flex temp and V speeds based, I think, on the ambient conditions and known runway length from the nav database.
The 777 (and other) FMCs do not take obstacles in to account when calculating the speeds - it just does a very basic balanced field length calculation. TOPCAT does take obstacles in to account.
There are many reasons why the N1 may not match up - the chief one that springs to mind is the limitations of the FS jet engine model.