The issue was perhaps more theoretical than concrete for many Southerners. They recognized that the Comp of 1850 gave them little if any benefit with the exception of a stronger fugitive slave law, and knew by 1855 that pop. sovereignty was going to produce more non-slaveholding than slaveholding states.
John C. Calhoun recognized before he died that if the South wanted to maintain its strength in the govt. it needed to concede the internal improvements issue, which was beneficial for many western farmers. They did not care about tariffs or banks, but they needed roads and canals and cut the deal with the commercial North to get them. Even Madison and Monroe while vetoing internal improvements bills thought they were a good idea with a const. amendment.
Overall, if these states could be somewhat integrated with the South, the general conclusion was that they would vote with the South on important issues and keep a balance of power at least in the Senate. Like you said, I am not sure if it would have happened that way, but Southerners understood they were a rapidly shrinking minority and wanted to provide a way of creating a political hedge against what they viewed as Northern aggression (the slave power rhetoric).