You can't overclock a modem.
What you have actually done is change your port speed (this is how fast your modem talks to your PC, and not how fast your modem talks to other PCs; on the net).
It doesn't connect at 115k, if it tells you that - that is because your modem drivers are reporting the Port speed rather than the actual connection speed.
Modems operate at a maximum of 56k, which translates to as someone said earlier around 7k per second max. Bear in mind to get that kind of speed your phoneline would have to be perfect - and you would have to live right next door to your telephone exchange.