Prof - I've read similar studies concluding that out spending your opponent does not have a significant impact. I tend to find them unconvincing but let's assume you're right for the sake of argument.
Lobbyists can still have an enormous impact. There's far more soft monwy spent anyway and it can have a devastating impact on a candidate. The NRA or the tea party can threaten to primary you if your voting record isn't up to snuff, forcing you to vote 100 percent for more guns or less taxes at the risk of facing a primary challenger backed by influential right wing groups or facing an independent right winger in the general that siphons off support and causes you to lose.
So what's your theory on why special interest groups donate money? Out of the good of their hearts? They don't want anything in return they just want to serve up their constituents cash for generosity' s sake?