Customs Duty and Logistics! Please Help!

I am moving to Qatar at the end of August and will arrive with my work visa, following which my sponsor will initiate my residency visa application process. I am also shipping a 20' container of personal effects and furniture into Doha by sea freight. I have received conflicting advice on the clearance of this through customs and would appreciate some advice. Is it correct that my shipment cannot clear customs until I have been in Doha for three days with my work visa and landed stamp? If this is the case, following my arrival in Doha on 26th August, when would be realistic to expect my shipment to be delivered to my home? Ideally I had hoped that my shipment would arrive at my home on 27th August but I have been advised that for this to happen I would need to pay 5% customs duty as I do not qualify as a foreign employee moving my effects for the prupose of residence until I have my landed stamp on my passport with my work visa (in spite of my sponsor providing a letter to customs). My shipping agent has also just quoted me an excess of £500 to ship my grand piano, plus 'port rental and detention charges at Doha' of £650 - am I right in thinking this is excessive??! Our shipment is due to leave our home in the UK next Tuesday so advice from someone who has been through a similar experience would be greatly appreciated. Many thanks!