The thickness of a radiator has a far less significant effect than the frontal area, plus in general thick radiators have a lower FPI so the cooling area is effectively very similar. Size and fan speed are the big factors as TTL was implying. (I think he probably missed the 'x3' after 290X though

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The 80mm radiator in the roof will give you some clearance issues, there's 90mm of room up there so once you take off a set of fans (25mm) that really only leaves you with room for a 60mm.
Logically the next thing to do is estimate the heat load from your components (which is basically the sum of the TDP (watts)) and from that compare it to the heat dissipation of a similar radiator at a fan speed you are happy with and then see if your delta T's will be acceptable. If they are too high ie. 15°C+ then you'll have to start thinking about how you can fit more radiators or increase the fan speed. There are quite a number of variables and it's all about finding the right balance.
I'm running two 780 Classy's and a 4820k with a pair of 360's both with push/pull fans, for daily use I run all 13 fans at 650RPM and generally in game get load temps around the 60°C mark but it's uber quiet. When overclocking which I only really do when benchmarking the Classy's will draw insane wattage, probably a total heat load of around 800watts so I generally knock the fan speed up to around 1000RPM to keep them chilly. I'd say it's very close to the right amount of rad space, for the setup, for me. I definitely wouldn't want less.
Just remember most watercooling pumps dissipate their heat into the loop, while it's not a big factor at the side of 3 graphics cards they need a respectable coolant temperature to keep themselves from frazzling. Anyway to me it sounds like your pretty close to the money but if you want it to be uber quiet all of the time then even more radiator area is the way to go.
JR