Is there anyway to tighten the balltop with the dustcover on without opening it up and voiding the warranty?
Not really.
That said, if it’s been more than 90 days since purchase, you don’t have a warranty left, anyways. Madcatz is still very supportive. Quoted from their website: (http://www.madcatz.com/faq/default.asp?Action=Q&ID=211)
“There is a limited 90-day warranty for our Street Fighter IV products. Opening or modifying the product in any way will void your warranty. However, if you run into any issues, please let us know as we may still be able to help you.”