Basically consider that the sync speed for 25 meg DSL is about 26.95 megs, so your customer is actually seeing 89 meg throughput on a ~108 meg aggregate sync. Overhead on VDSL2 seems to be roughly 6-7% (going from benchmarks here, not doing the math). ATM was one of the biggest sources of overhead with ADSL, and that's gone now, hence the lower overhead. So if he's seeing 89 meg, that's about 17% overhead, way more than we'd expect to see.
Although there is also some MLPPP overhead too, of course, but that was like 0.4%.
Developer: Tomato/MLPPP, Linux/MLPPP, etc »fixppp.org