The benefit of going with that card is the availability of Mac drivers. Although the one that i just ordered is showing absolutely abysmal transfer speeds in a Mac Pro - about 2 Gb/s. I also find that it's somewhat picky about transceivers; it refuses to recognize any of my FibreStore transceivers.Take a look on eBay for Solarflare brand cards, they're very good. SFN5122f for instance.
I have not contacted Mellanox yet. Will probably try on Monday although it is a holiday so they may not be available.. Guess other people who ordered the bracket will have to confirm what part # they get and if it fits properly.