so the higher weight can apply as long as the climb gradients are still good to avoid terrain on climb.
That will be the issue. While the AFM may say "OK" at the airfield elevation, the final altitude of the Missed Approach will be the issue. If the final terrain clearance altitude/MSA is very high, the weight may have to be lowered to ensure the aircraft can get there in the event of an IMC missed approach. The weight to achieve 2.5% (PANS OPS MApp gradient) at 2000ft AMSL will be higher than that to achieve 2.5% at 5000ft AMSL.