I currently have a 140mm fan resting on top of my GPU, facing towards the NB heatsink and the CPU heatsink. Then I have a 140mm fan on top pulling the air from the fan on the GPU exhausting out the top while another fan exhausts out the back of the case. Seems pretty efficient but I've always been curious if I could get better results with the Megahalems orientated vertically
Comparing vertical versus horizontal, one needs either a top-panel fan exhausting the horizontal mounting, or a duct-box ported to the rear exhaust fan. In the latter case, there is little appreciable difference between a DUCTED vertical mounting and a DUCTED horizontal mounting. I posted my test and proof of that around 2007.
For the 2007 case, for the horizontal mounting, the duct covered the two broad sides of the cooler in addition to the duct end ported to the exhaust fan.
Ducting makes any directed airflow more efficient, but it also requires an investment in time, effort and tedium -- up-front. Even so, it's a pretty cheap mod to make.
In your case and from your description, there should be plenty of airflow for those two fin assemblies at issue. But you could still wring several degrees C in cooling from them -- with, as I said, the tedious effort.
In parting, I'll say this. For your rig, I might consider at least a couple alternative configurations. If the intake airflow is sufficient, I'd investigate two fans pulling air from two sides of the horizontally-mounted cooler: rear exhaust and top exhaust. For simplicity, though, I'd consider the use of only the rear exhaust fan, and I'd probably block off the top-panel vent after removing any fan from it. I'd extend the duct box to cover at least half the widest dimension of the heat-pipe tower, and I'd pick a beefier rear-exhaust fan. I'd still consider the duct-vent to pull air from the motherboard, or the extension of it to cover more motherboard components.
But it has been my experience that no further effort per what you call the Northbridge heatsink and fins is necessary -- IF -- there is sufficient case intake and a decent amount of pressurized turbulence in the case. The fan blowing down on your GPU provides plenty of cooling for those fins.
You could eliminate that fan, though, with the sort of mods I've described. You could even use a smaller fan instead. For instance, you might want to look at a 92x15mm fan if you can find it. In your case, the 140mm fan over the GPU adds some clutter. In other situations, adding a smaller fan at some strategic position also raises a need to thermally control it to eliminate noise as an unpleasant by-product.
I've got two GTX 970's in SLI. I had initially over-clocked them, but driver-installation problems required me to uninstall Afterburner -- the OC'ing utility. Afterburner also provided custom thermal fan-control for those cards. Now, I think I can reinstall AFterburner, but I was thinking I could build a duct-box or duct-panels for the graphics cards, and put a 92x15mm fan in the mix at the front-most edge of the cards -- blowing air into the box. There are a lot of incidentals, details and complications to that, and obviously the assembly would need to be both solid and easily removed for maintenance.
But the best choices for mods are the simplest. For instance -- the simple duct-box with the slot cut in it to use the Bernoulli Effect to draw air off the motherboard. Better than adding another fan, and as simple as four cuts with an Xacto knife.
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