Is it possible to mod a class 2 bluetooth device to work as a class 1 device?
You need a PA (power Amp) before that Antenna. Not an easy hack but possible, it will be illegal though. (you need to get a new FCC license for it)

If you have an SMA connector, you can build a tiny board with a PA and give it a shot. PA2423 is an example of such power amplifier. 

The best way, replace the whole module with a class 1 module, probably some software work is needed but will be easier, cleaner and potentially faster.