The performance of an OC'd powercolor 7870 LE (or sapphire's 7870 XT) is on par (sometimes better) with a stock 7950. CF'ing a 7870LE with a 7950 would in effect give you a CF'd 7870LE setup. Before attempting, you should see if your MB is compatible and if your PSU has enough amperage in your 12V rails as the 7870 LE uses more power than a 7950 (200W v ~180W). Including OC, you would need at least a combined 40A along your 12V rail (>600W PSU).