Welcome to GTA Steve, nice to have with us. I have 3 GAMO Shadows and they are indeed strong shooters. Recently one of mine went from it's normal mid 900's to 750 fps. I pulled it apart and found the seal had to be replaced. I replaced the seal and it's now back up to full strength but I have to say that when it was shooting 750 it was still extremely consistent and just as accurate, so if you tune it down it will indeed still be a nice shooting gun. As for what spring to use to tune it down I wouldn't know which spring to suggest but I'm sure one of the members with more knowledge will be able to suggest a suitable spring. I guess you could also cut a coil or 2 off the stock spring but I'm guessing here.
Anyway, again, welcome and I'll be looking forward to your future posts.
Jeff