In short terms, yes, stiffer springs would help with this issue.
However, when looking at the bigger picture here and taking everything into account, then no.
These are the factors that come into play in regards of the max weight limit:
- Suspension; Stiffer/better suspension that's capable of heavy load is needed.
- Brakes; Potentially bigger and better brakes (Not sure how much pressure the Rebels brakes can handle, but guessing they are "maxed" out to match the original weight limit. And yes, better brakes does not give you more weight limit, but you would like to stop in time, just in case, right?)
- Stress points on the bike; Such stress points can be everything from the rim, swing arm, frame itself, etc, etc. Basically every place on the bike that will be strained under heavy load. And these might end up needing reinforcement to handle the load over time. For starters, I believe the original support that holds the back fender, and thus the passenger seat, would be one of the first things to buckle under stress.
- Tires; A tire isn't just a tire. Often, a tire might be what is "mainly" deciding what the load limit actually is. Now remember, read "mainly" Everything else mentioned here is highly important as well, so just changing the tire to one that handles more load doesn't mean that you're good to go in the long run. But it is a big factor in regards of the max limit, as all the weight will end up down into the tire either way you look at it.
I also recommend that you take a look here:
It's a good read in regards of the topic here, you might get some new "aha" experiences.
Best of luck.