Since GSM Shield uses SoftwareSerial
The shield does not use SoftwareSerial. The library (stupidly) uses a variation of SoftwareSerial to communicate with the shield.
Bend some pins on the shield, add jumper wires, and rewrite the library.
Does this make the performance any better?
HardwareSerial always beats SoftwareSerial. Whether the difference is noticeable depends on how much data you are sending/receiving and what baud rates are involved.
Sending one character a week will not demonstrate a measurable performance improvement when using hardware serial instead of software serial. Sending a million characters an hour will.