But the incumbent and non-incumbent limit is the same, so how would raising pay help?

So if right now the congressional income is 300K for two years (fuzzy math, the numbers vary I imagine) then incumbents and non-incumbents alike can only spend that much. If pay is raised to 400K for two years, then incumbents and non-incumbents are both given the new limit of 400K. I don't quite understand what you are saying.