Flying Croc....
On a normal day, the A380 will take a full load of pax and cargo without any issues to the USA west coast. We do it now to LAX with plenty of room to spare. Sometimes up to 20-25 tons on MTOW to spare which could be used on fuel if wx conditions required.
IAH is about 1-2 hours longer then LAX (worse case) so the answer is yes, they can carry a full load. We will just have to use this spare 20-25 tones on additional fuel for increased flight time.
Having said that, if the wx is dodgy on arrival, which requires additional contingency, and you happen to be maxed out on that particular day, then yes, you "may" have to take a hit on payload. But that's no different then a B777. Couldn't tell you how many times thats happened to me over the years on the B777. Not a dig, just the way it is.