If you were running down at 20Khz, it probably does not matter, as you will be saturation limited, not core loss limited.
The N27 material is now pretty old, and it still works fine for many lower frequency applications, but the newer N87 has lower losses at higher frequencies. The higher you go, the better N87 becomes.
As you go higher in frequency the losses increase, and you will need to reduce the flux swing to limit temperature rise.
There is really no fixed upper frequency limit as such.
You just need to drive it less hard as you go higher, and larger sized cores with greater internal volume will reach higher internal temperatures, everything else being equal.
A lot also depends on the expected ambient temperatures reached inside the equipment, its all more a judgement call than anything else. In the end it probably comes down to running some practical tests.