Believe it or not, Dave Welham used to get the Relum Tornado and Zephyre's working for FT. I had one as my first Air rifle. I think Dave used to change the spring and use a nylon or PTFE washer and a few other things and they shot very well indeed.
I think that he could actually get more power by removing the inner spring. If you want some advice on doing it up, give him a call.