lets break this down:

winter(N,D,J,F) solar insolation avg. === 2.2 kwh/sqmeter/day
multiply by 317.1 ==== 697.6 btu/sqft/day
multiply +/-50% collector efficency === 348.8 btu/sqft/day
multiply by collector area (300sqft) === 104,640 btu/day
divide by 8hrs of sun (9a-4p) === 13,080 btu/hr when sun is up

What is your avg. heatloss during the day for N,D,J,F ? If it is close to 13,000 btu/hr , you could reduce your winter heating bill by 1/3rd and have plenty of hot water for domestic use during the shoulder and summer months.
Between electric radiant heating & electric domestic water heating, plus the 30% federal tax credit, this system could have a fast payback period.
If you plan on staying in the house for the life of the system, it makes sense to INVEST into this kind of energy savings.