So my biggest complaint with Metallica LE over the pro has been the square plate vs the captive ball. So I decided to just order a newton ball and swap it out. The results are great.
The sparky shot is slightly easier to hit from the right flipper now. The snake is way easier to hit now that I can reliably bank off the captive ball into the snake mouth. As for locking balls into the coffin, I notice it being no harder. The captive ball sits slightly further back. And shots to the plate weren't always a lock anyway. It feels about the same as it did before with the plate for ball locks.
In short, I see no downside to this change. Only upside.
For anyone that wants to do the same thing, you can get the newton ball from pinballlife. http://www.pinballlife.com/index.php?p=product&id=697. To install it, you just lift the playfield and remove the 2 11/32" nuts that hold the plate rear corners in. Then remove the 1/4" screw and pull the old square block out. Insert the captive ball in the front hole. Now you need a 3/8" nut and a lock washer. I couldn't get a nylon but to tighten so I used a washer (one I removed from the block), lock washer, and regular 3/8" nut to secure the ball in place. Lower the playfield and enjoy