In the past decade we've had 3 mild/moderate El Ninos (most likely scenario for 2009-10). 2002-03 was just slightly below average at Sunshine but with a very slow start. 2004-05 was a disaster for most of western Canada but at least Sunshine in its cold climate didn't get deluged by the January rain. 2006-07 was an excellent year at Sunshine including unusually early openings of its extreme terrain. 2006-07 ranks #13 in El Nino strength of the last 43 seasons. Sunshine has no above average seasons in the 12 stronger years.
The bottom line: Sunshine will likely be good enough during its March/April prime in ~75% of the El Nino years. But I would defer booking until adequate snow is on the ground.