GT Distributors had their Glocktoberfest this past wekend with Glock armorers on-site. I took it for a visit. They are pretty much in line with what you said. They said once Glock replaced all the springs, etc, the 27 is running in top notch mode - but the older mags are not. They recommended rebuilding the existing or purchasing new. I just purchased a couple of new Glock 22 mags for backup mags. With the sale they had going on, it was the same cost as rebuilding existing. And they said that is the reason Glock tossed in a new mag - because people typically have issues with older mags. I have not been to the range yet to prove everything - but will get there in a couple of weeks.
They also stated all mags that are relied on for personal safety should be rebuilt with new spings & followers or replaced with new mags every 3 years - this, assumed they are kepts loaded or are being loaded on a regular basis. Learning more everyday.
Stand by for range report.