Wit 9 sq inches per watt, you get a thermal resistance about 20 K/W. Plus the thermal resistance of the LED package itself and any aditional dielectric layer (a thin film of thermal adhesive is 0-9-1.3 K/W), it put the thermal resistance of the system near 30K/W. Although its enough to keep decent efficiencies, it may result on unaceptable LEDs duration when running LEDs hard:

At 700mA, a blue leds draws about 3.5V or more. Still at 3.5V, its 2.45W. At 30 K/W, it result on a increase on junction temps of 73.5ºC. So with ambient temps around 25ºC, the LED chip is going to work near 100ºC. Its good for many applications, but on ours, with many hours on a day, that result on shortened life. Not dramatically shortened as if Tj were 150ºC, but shortened.

Most manufacturers only "guarantee" (based on extrapolating short term measurements by stadistical analisis) a lm mantenience of 70% at 50Kh when Tj is kept at or below 80ºC. For the typical depreciation curves, its about 35Kh for 80% of initial output, which mostly is the criteria for horticultural lamps replacement (while human barely notice a drop below 30% on light intensity, for plants it means at least 30% less production, that is usually inaceptable).

For Tj over 80ºC, lm degradation is exponential with the increased temperature. So Tj about 100ºC are aceptable for many applications, where 25Kh until reaching an emission 30% lower of initial is aceptable. But for ours, if you want to recover initial higher investment by electrical bills savings, we want a lm manteinance of 80% at 30Kh minimun, and for that is a must to keep Tj closer as possible to 80ºC. It requires a thermal resistance from chip's junction to ambient temp of 22 K/W.

So when using 9 sq inches per watt instaled of heatsink dissipating surface, if you run your LEDs at 700mA you get a somewhat reduced expected useful life of the array.

Probably the use of airflow on the heatsink will help reducing further those 20 K/W of the same heatsink on open air. So probably with it, you are going to be able to expect 30Kh of useful life. But i would choose to run LED slighty below 700mA to guarantee it (about 600mA). Aditionally, it enhances LED's efficiency.

When running the LEDs at 350mA, that heatsink is more than enough, without the need of forced airflow.

Any reason to go with the Crees over the Luxeon IIIs or K2s?
About blues and whites, way better energy efficiency. Currently Cree is doing much better blue leds than Luxeon (as better as double light output per watt).

For red leds, benefit isnt so clear. I prefer the Crees over the Luxeons because the reduced Vf. But differnce on efficiency, although still better for the Crees, is small, and the easy avalaibility of Luxeons and price (when buying small amounts and not full reels) often does the Luxeons a good alternative.

One think you must take into account when using Luxeons and controling voltage, and not current, is their tendency to reduce Vf for same current after some hundred (or less) hours of operation. It mean that you design the system initially to run, for example, at 600mA, but after 200h, its running at 750mA.

Curently, im going to try the Seoul SemiConductor P4 red leds. If specs are correct, they are way more efficients than both Cree and Luxeons (always talking about top bins avalaible: M for K2 and S for Lux and SSC).