Composite will be much superior then Al because it does not fatigue. So that's why maintanence interval of the 787 will be much longer then comparable Al aircraft. I think from what I read from Flight International, Boeing claims that 787 will have 30% less maintanence check then a Al aircraft... Also, I don't think Boeing will over engineer the aircraft because airlines might worry about the use of this relatively new materials, if they over engineer their aircraft it will make the 787 not competitive over A350...
What Boeing is doing though, is that they are going around to different airlines and doing many demonstration to prove to the airlines that composite is tougher then Al... They are also showing airlines quick repairing methods is possible... But one of the things to be work out will be moisture within the composite structure... especially on honeycomb composite structure (on the tail)... as you can see from the vertical stabililizer falling out from the A300... once water manage to get into the composite stucture, it will soak it and weaken it very quickly... but I am sure smart engineers at Boeing will work that out for sure
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