I'll see what the stock fans are like at the front. I have four spare Gentle Typhoons to locate around the case if necessary, so I was going to either replace the front fans with three of them or leave the front fans in and tie a few of the GTs to the back of the hard drive cage. This will depend on the amount of noise generated and the airflow of the stock Lian Li fans (I've seen conflicting reports depending on what model fan is included). I'll try it with the hard drive cages left as they are to start with and see how it goes -- as you say, they look pretty well aerated.
I'll take your advice and try without the rear fan to start with and just remount the fan grill. I've also managed to track down a 140mm Lian Li mesh grill for one of the side holes so I'll try an exhausting fan opposite the video cards to see if that makes a difference (since I think the Windforce cards mainly exhaust outwards).
I'm also planning to put a GT as an intake on the top in front of the CPU heatsink, similar to that CM 690 II diagram.
Will be a few more weeks before I can get it all set up but I'll try to take some pics and do some temperature tests when I do so others with similar cases might have a better idea about the best fan configuration.