According to this link
http://www.b737.org.uk/powerplant.htm#Engine_Ratings You will only get fullrated thrust.
However when the EECs are in (hard) ALTN mode, you will be able to overboost the engines.
Copy paste from a 738 FCOM
If the EECs are in the alternate mode, advancing the thrust levers full forward provides some overboost and should be considered only during emergency situations when all other available actions have been taken and terrain contact is imminent.
So what do you get when you firewall the engine on a RYR aircraft (in hard altn mode)? Don't know - a bit extra thrust I should think.
