Yep, the rep is right. Systems are rated by the sum of their parts, and not all combinations reach the top efficiency numbers - which is why Trane's website says "up to 16.00" for the XL15i and "up to 16.50" for the XR 15i.
I'm also looking for a heat-pump and in my first quote with a Trane dealer, the options seemed to be limited regarding qualifying systems in the 4-ton size. The Lennox rep I found had no issues finding a system that qualified, though. Got another Trane dealer coming by today, so we'll see if he comes up with any other options.
BTW - Why are you preferring the XR15 over the XL15i?