Recoil spring tuning questions, help?

[Twilk73]

I have a strike one sai version and I been tuning it for competition.  
 

So I’ve tried everything from 15# down to 12#. I like the way the gun cycles with The 12# spring but I’m nervous the slide might be slamming a bit too hard and with the giant window cut in the top I’m nervous the slide will crack eventually. Also the gun cycle’s with All three springs but if the gun were knocked out of battery with any thing other than the 15 pound spring while loaded I have to push the slide forward. Will this be a problem! I know I wouldn’t want this on carry gun but seems like it shouldn’t be an issue with a competition gun unless I were to jam a barricade or something. Basically if I do a half a press check only the 15# spring pushes the slide forward. 
 

I’m shooting 147gr 132 power factory loads. 

[Postal Bob]

This is a 9mm I'm guessing? Most 9mm are using a 10lb spring, so a 12 lb should cause no problem.

[Twilk73]

Cracked slide

 

So my new gun is similar to the one above that I cracked the slide on around 6800 rounds. The slide is cracked on both sides. Gun wasn’t dropped. I believe it was cracked from stress from recoil. Anyway, my new gun has a large windows up top. Essentially the new gun is likely weaker. I’m trying to avoid cracking the new slide too but maybe it’s unavoidable considering where the crack is the slide is real thin there. 
I guess some saving Grace is that the new gun slide cut isn’t where the old guns crack formed. 
 

New gun