Mercurys and Airsporters often suffer from 'O' rings which are too tight a fit in the compression chamber bore. The cure is to fit the smaller 'O' ring from the BSA Meteor, or if you have a collection of 'O' rings, one which is not such a tight fit. Also, the 'O' ring has to be in good condition, it is easy to knick them as they are fitted. Use old-type camera film to ease the 'O' ring in. If the spring is in good condition and the piston seal is sealing and a good fit, you should get between 10.5 and 12 fpe depending on pellet from an Airsporter in .177. I think the later S model increased power by using a steel piston head - less bounce with a stronger spring perhaps. As you say, it might be an idea to try some other pellets, JSBs, H&N FTT and RWS Superfield would be my first choices.