Well took her to the range today and here are the results:
With Gamo Rocket 14.5 gr a average of 926.75 FPS at 27.66 foot pounds
With Exact jumbo heavy diabolo 18.13 gr a average of 841.5 FPS at 28.51 foot pounds
Advertised FPS was 1050 with 15 gr pellets. I mentioned in the original post that it was loud but noticed today that only the first shot was loud, the rest quietened down considerably with the silencer on
Accuracy - need to bear a few things in mind - I had just shot out 200 rounds 9mm. The mozzies were killer today, stand still and they swarm. And I had a cheap shit scope on with aluminum mountings and haven't had a chance to install a stopper so scope moved back with each shot. Rifle is brand new, unserviced and hasn't been worked in yet. After 10 shots the scope had moved back 25mm so the groupings reflect that and are not bad considering.