Hi Dreamland
Yes you will still make good the contraint FL180 at XYZ.
However the flown profile will be the original 280kts profile. Flying 320 kts selected will have you flying with quite a bit of thrust. IMHO better to forget the profile and go for V/S initially to shallow descent and then go for OPEN DES.
Re-inserting cost index or anything will not change the descent profile to match the selected speed. It will only recalibrate the profile using actual OAT and actual w/v but still assuming the 280kts since this is the managed speed target.
Apologies if I am not clear, but English is not my native tongue.
S