I'd dispute your statement that SEACOM has been the only one with a noticeable impact. SEACOM did bring the price down as a first start, however, the impact of competition with WACS and EASSY then in turn brought SEACOM's pricing down to a lesser degree.
Also, without WACS and EASSY, the outages that SEACOM has had would have been felt far far more by the average consumer. The "noticeable" impact you seem to miss, is found in the fact that you don't see as many outages.
With regards to SAEx, while this is an interesting cable system, the pricing is going to be rather interesting, since the cabling between brazil and the states isn't particularly cheap at the moment. The other thing that will need to be watched is the impact on the pricing on this cable when its competition arrives in the form of the *MUCH* larger WASACE cable (WASACE has a design capacity for more than 3 times the SAEx cable, and is built in a ring topology covering Africa, South America, North America and Europe).
Having looked at both SAEx and WASACE fairly closely, I would be far more inclined to place my eggs in the WASACE cable at this point (though admittedly, both are still far from being completed).
Andrew