Anyway this system is pretty slick and I'm running right now Prime95 for over 1 hour @ max temps of 77, 76, 74, and 73 on the four cores.
My oc setup is:
base clock: 191 x 21
dram core: 1.35
dram bus: 1.65
I chose the H70 over the Noctua for pure asthetic (IMO) and my OC goal was 4.0Ghz which I have successfully hit. I do think i could hit a better temp range of -5 degrees hopefully with a better airflow design.
As you know the H70 recommends that you take the fan and reverse it so that the air outside the case is cooling the rad. I did just as it recommended but I only made the following change...I took the spare 120MM rear exhaust fan and mounted it to the bottom of the chasis directing cool air to the top of the chasis. The way it is set up is that everything is an intake except the 230mm fan on the top.
My son is running a 920 using the R2E and he has three main BIOS profiles. The 24/7/365 BIOS profile has temps from 55-58C at 3.78 GHz prolimatech Megahalems) ... that he uses for homework, web browsing. He has a 4.0 Ghz "gaming profile" that runs at 69-73 and another at 4.2 that iirc, runs about 70-74. The 932 or HAF-X might a helped a bit.
As to the "spare fan", I assume you mean that you took off the rear case fan and installed the Corsair fan for the H70 and then installed that i the bottom bracket. As long as there's nothing blocking air access under the case, it should work but I'd be concerned about dust with that intake location...perhaps try the side door location and see if it changes anything.
Front: 200 mm fan x 1
Top: 200 mm fan x 1
Rear: 120 mm fan x 1 )Corsair)
Bottom: 120 / 140 mm fan x 1 (optional)
Side: 200 mm fan x 1 (optional) (can be swap for 120 mm fan x 2) - you can also install these options
Another route would be a better cooler .... understanding that you gave up the better air coolers for aesthetics, that does mean giving up some cooling capability. The Antec Kuhler 620 review on benchmarkreviews.com showed that, when fitted with a Delta 113 cfm fan, the temps dropped 3C from the stock H70 fan. That might be the easiest solution to your concern. I should not however that the Kuhler 620 was another 6C below the H70 with the same Delta fan.
I would think than that you could get 74, 73, 71, and 70 on the four cores with the fan change and 68, 67, 65, and 64 on the four cores by swapping out to the Kuhler 620 and same Delta fan.