Too late...
You can already track anything with an IMEI - products are already in place within the UMTS/LTE operators operations centres which do that so that they can easily see where network congestion is happening, or track VIPs, or a variety of other use-cases for performance monitoring and optimisation. Assuming the IMEI has been correctly registered (and not spoofed), you'll be quickly able to see which are your problem devices with a simple filtering of the data against your TAC-list database (operators can get one from these folks, for example: http://www.tuv-sud.co.uk/uk-en/about-tuev-sued/tuev-sued-in-the-uk/tuev-sued-babt/other-services/imei-number-allocation/imei-database).
Even if the IMEI is spoofed to say "I'm an iPhone 6+", it would still be possible to detect the general performance of such phones and look for outliers in the data - either highlighting a faulty device, or a fake device.