I recently traded in my .40 S&W Shield for a 9mm Performance Center Shield. The Performance Center trigger is better than the stock Shield trigger, especially before break in,  but I was wondering how it compared to Apex's Shield trigger (Brownells). If I decided to go with the Apex, I'd go with the full action enhancement kit.

Is there a noticeable difference between the Performance Center trigger and the Apex? If so, is that improvement worth $152?

I have a PC full size l40 and a shield with the full replacement Apex parts with the metal slanted trigger.  Both are different and both have their issues. Again, I have the slanted trigger and not the flat face Apex trigger on my shield. However if you did not pull low on this trigger, the trigger safety will not disengage completely. This prevents the trigger from firing. Apex may have corrected this issue but I'm not sure if this is the case. The trigger safety is not like the trigger safety on the HK vp9 which disengages from the middle to the bottom of the trigger

The apex is crisp and has a far more tactile reset than the original trigger and the PC 40l.  The apex trigger had a lighter pull than the original trigger and the PC 40l at first: Now the PC trigger is just as light as my Apex shield trigger after shooting the hell out of it but is not as crisp has the apex due to the hinged trigger.  I probably will not replace the trigger on my pc40l with the apex because of the trigger safety issue and the small difference in performance.  It just is not worth the $150 extra in my opinion.

With that said the apex trigger was a big jump from the original standard shield trigger but not so much with the PC model after shooting the hell out of it.

YMMV