"A cantilevered design(405) is simply not going to offer the same level of rigidity as a monoblock design(400)."
Fair call, but since I have a B.A in architecture I will point out that they use cantilevered designs for bridges, cranes, buildings, aircraft and motorcycle chassis. There is nothing wrong with the design philosophy that manfrotto have applied in the design of the 405 head the gearing is precise and it doesn't exhibit any detectable "creep" or slippage in the mechanism I have a friend who works in industrial design and he was rather impressed with the Manfrotto 405. As you said in a post above perhaps more monolithic structure might suit you better but the main advantage of the cantilever design is using strategically placed fulcrums to balance and enhance the overall rigidity of the structure and in obtaining this goal I will say that Manfrotto has been quite successful. The Arca swiss cube only weighs 900grams and the Manfrotto 405 itself weighs a hefty 1.6Kg - I would wager that the heavier head is the more effective at dissipating and absorbing any vibrations it will be, and with the larger area of the RC4 platform which gives the camera a physically larger area of contact which further spreads out and dampens vibrations. There is a Korean knock off version of the Arca swiss cube called the multiflex that might be cheaper and still provide the quality you need, However I consider the Arca cube to be a bit of overkill unless you are using Large format with High resolution digital medium format backs. For 35mm and smaller self contained medium format digital systems the manfrotto 405 will be perfectly adequate even for longer telephoto lenses. I will also add that the B55 from RRS was not able to pass the glass of water test I subjected the Manfrotto 405 to. As I see it the mass of the camera has to also be more or less matched by the tripod head, and the legs have to be substantial enough to support both. You can have the best set of tripod legs in the world, but if you have for example a manfrotto 488 on the tripod you can kiss any hopes you have of obtaining decent stability goodbye.
Also I do have to ask what photographic equipment are you using that makes the arca swiss so appealing to you?