I have a post on here that will at least help improve your trigger . " Beeman 1358 Tuning and shooting". Your 1357 is pretty much the same except for caliber. I know the 1358 has a cliff shot curve using the stock spring. It is suggested to clip one or two coils off to get a bell curve. I clipped one coil off mine but you still have to have the HS backed almost totally off to get a good curve. Maybe the 1357 is similar. I also purchased a lighter spring that I have been testing out to get the best curve to my liking. Lane can make custom regs but you are going to be better off getting a gun that comes with one so everything is working in unison.
On the GTA site there is a person that has bottled his 1358. You have to be real experienced and have the capabilities to make parts if needed. Like Vetmx stated you should have probably bought a different gun with the features you want. So between lightening up the HS and getting the trigger better that's about probably the most you can expect for it.