Like Denny said, it makes some gains at high rpm, and may be good for a racing engine. If you're changing the cam anyhow, it might be worth the money. Isn't a 4-7 swap though, instead of a 5-7? 4-7 are matching cylinders.

From the Crane website:
The 4-7 swap firing order can help intake manifold distribution inequities, reduce the load on the rear main bearing, and perhaps gain torque and horsepower in certain RPM ranges. Although some drastic claims have been made for this modification, a power increase of 1 to 2 percent is realistic in some situations. No machining is necessary, just slide the cam in, swap some spark plug wires, and there you go!

As an aside, Chevy switched the firing order on the LS motors. They swapped 7&2 for 4&3.

1-8-7-2-6-5-4-3 LS motors
1-8-4-3-6-5-7-2 Std Chevy Firing Order