You can specify a limited MTOW of 1999kgs if you wish. I fly a Seneca III which is artificially limited to 1999kgs compared with a practical maximum of 2156 kgs. The impact on this is it limits the options of payload vs fuel. It means that really 4 up including pilot is the maximum with enough fuel to get anywhere. The extra 157kgs can be a real help if you need extra payload.
I suppose you have to decide how often you will fly IFR and be subject to Eurocharges compared to the number of times you will need that extra payload.
You will need a new C of G schedule, ZFW, MTOW and all that mullarkey to be reflected in the AFM and any loadsheets you use.
Despite all this, I don't know exactly how you go about it with the CAA.