Is that mod production/SSP legal?
I'm sure it's ESP legal, they'll let a 2011 in that division.
Mike's internal magwell blend is SSP legal.
As for cracked Glock slides, I've only ever seen one. It had gone full auto in a previous life and eventually cracked below the ejection port. As for the plug making slides more prone to cracking, well let's just say I don't trust mfg's reps any farther than I can throw them, and that goes double for Glock reps!
I'm not holding a lot of confidence in their comments as well. By the way, at IDPA Nationals, one of the Glock Reps stated that Glock will no longer provide night sights in future guns. I thought that this was interesting. Of course, one can always install aftermarket, but certainly is a cost savings when provided by Glock.