You actually can not, as the 7870 Tahiti LE only crossfires with a 7950/7970. The 7870 LE is actually a toned down 7950, using the Tahiti architecture instead of Pitcairn. Its not a beefed up 7870 like the misleading title presumes.
Crossfire doesn't work that way, the higher card gets capped at the lower cards specs so even though you CAN crossfire a 7870 le and a 7950 you would NEVER want to. Raising the clock doesn't really have anything to do with micro-stutter.