Whether if you would enjoy the full S$800 per year rebate
depends on the size of your car's CC. If you got a small car like
mine, you will only enjoy S$400 rebate because my car's road tax is
~S$450, and the OPC road tax is at least S$50.
Secondly, if you count in the 17k rebate for the weekday voucher,
you might as well get a normal plated car. So what my friend did is
compared the saving from the road tax and concluded that he have 32
coupon to use per year. These would be a more convincing
comparsion.
Other than the admin fee to convert your car to normal plate from
OPC, the rest is petty much what you indicated. 