I have tried it on my cocker and the balls kept rolling out, so nothing there for the first shot. Getting a Python size kit will stop this from happening though.
I use an APEX on my Viking and Mini and they are great. There is a huge range increase with the Apex. Unfortunately the accuracy suffers and the low velocity means ball will usually only breaks if you hit something hard, i.e mask, hoppeo, gun, etc. It is not a big issue with a fast electro, but on a 'cocker may be a big down side. Also I taped up the porting right behind the plastic end. It got me between 10-15 fps more in velocity. And the Apex is LOUD!!!! Sounds like a cannon the feild.
You might want to check out the LAPCO version
A friend got one at Monster game this year and It is not only more accurate, but is also a lot quieter. I think it is superior to the BT APEX.
For saftey you have to crono the APEX with the spin adjustment all the way back(least amount of lift on the ball) so you dont excede your fields max velocity. I have tried my APEX up to 295 fps, and found the accuracy was very erratic. I found that at 250-260 fps it was much more consistant.
Hope this info helps you.