Very odd indeed! Now I can certainly understand the confusion!
I just cross referenced the product number of the Sapphire card on the site you linked, and on a number of other sites the specifications of the card with that product number are listed as they are on Sapphire's official site (1000, 1100, and 6400). That combined with the fact the clocks on the site you linked don't line up with any 280 Tri-X on Sapphire's official site leads me to believe it's a rather unusual typo. The alternative would be it's a very odd edition that's only sold in a few places.
I had a quick dig around, and there's at least one other Sapphire card with a mistake in its listing:
http://www.pccomponentes.com/sapphire_r9_280x_vapor_x_uefi_3gb_gddr5.html Either the boost clock is wrong (listed as 1000 MHZ as opposed to 1070 MHz it should be), or it has the wrong image, and should be one of the 3 fan Vapor-X models (Sapphire certainly make their model system convoluted!).
So, in conclusion, my suspicion would be that it's a typo, but just in case it isn't, the answer to your question of how much difference 50 MHz would make is very little, 2-3 FPS at most (it's less than 5% slower after all). Hope that helps!