If you're fielding the Razorwing in a dual role, ignore the Splinter Cannon upgrade. Once you're fired the missiles you should be directing the lances towards tanks every turn, rendering the SC useless. If you find yourself in the situation where there are no tanks left to shoot, chances are you're close to victory anyway and the Venoms will be mopping up the stragglers.
For a pure AI Razorwing, the SC will double the number of shots you can fire (ignoring missiles). Those extra shots might not hit as hard as the cannons, but much like the Venom the volume of fire (especially when half of that volume is AP2) will grind down units. It also makes you slightly harder to glance to death (note: looks likely to change with 6th) as you can take another weapon destroyed result.
If the 6th edition consolidation changes are true, it will make WWP lists slightly more viable. Units that come out of them can take out vehicles in close combat and then consolidate into the passengers, meaning no round of return shooting. IMO it will still need reserve manipulation to ever be a viable tourney list, but for not quite so serious lists or having a couple of units coming out of the portals, it will boost it loads.
The unit that will be buffed the most by this is Beastmasters who have a 24" threat range and can rend tanks to death