what beats the Tahiti LE's ? , that chart is for the standard 7870
270x beats 7870 LE;
http://www.kitguru.net/components/graphic-cards/zardon/amd-r9-270x-review/7/
7970LE gets a marginal win in Tomb Raider, 270x either wins or ties in everything else. Uses less power too (important for heat in Xfire), and you would get your own warranty.
I would get new 270X's over the 2 7870LEs you've been offered, personally, for the extra £7 each