Ok I spent a little more time researching this (which as a reviewing site I would have expected you guys to do instead). The Sapphire Tri-X does not have a "Quiet" mode or performance mode switch on the card. Instead it is a UEFI Boot mode switch. However, you stated at the beginning of the review "All of the Radeon R9 290X cards are set to their Quiet Mode firmware setting" which in the case of the Tri-X does not exist. What this means however is that the Asus card in quiet mode is set to allow its temperatures to rise to 94C. By nature of doing a comparison at this point you should have the other cards to their performance settings which is equal to the default Tri-X setting.
Secondly, Sapphire lists the Tri-X on their site: as a 2.2X which is the same as a 3 slot card. You can't put them back to back (not that anyone would do so with a non-blower design).
http://www.sapphiretech.com/presentation/product/product_index.aspx?cid=1&gid=3&sgid=1227&pid=2090&lid=1
Most importantly to me your results do not match the temperatures I am seeing on my existing card, in my existing setup. I have an Antec 1200 which is full tower case, but the fans are all stock and the case is not known for exceptional cooling. I do not think the results of your review are representative of what someone buying this card would expect in a normal air-cooled case.
As far as your assessment of the heatsink, have you ever seen a CPU heatsink that exactly matches the size of the CPU? No, they're all much larger (including the heatsink on the Tri-X) and that doesn't make the heatsink bad, it indicates waste on the part of the design that is contributing to the cooling of the card less, but in this case more heatpipes than come in contact with the GPU doesn't translate to the design being bad. I'm not saying the Tri-X cooler isn't superior, I'm saying labeling the Directcu cooler as "Too Hot" when you have it set to a setting that is DESIGNED to operate hot and"Relatively loud" when it is far quieter than the reference design is not very objective.
Ultimately the Asus DirectCU is overclocked higher than the Sapphire Tri-X so it is going to run hotter. There is just too little information about the cards and their different settings given in this review to make it particularly useful.