This is fascinating! I've been down my parents' street and most of the neighbours are getting 8-13Mbps, so perhaps a master splitter would be a grand idea for Mum & Dad - they could triple their speed. Interestingly, one house (which is super dodgy - it made the news a year ago when someone let off a firearm in it and then sparked a police chase) is getting 17Mbps which shows as exceeding their maximum available speed. Now I'm going to check my street.
Estimations are exactly that. who knows if they are tweaking, Have a perfect line pair or simply are lucky.
your parents could be on the unfortunate situation of having a really bad pair.
For my address it pulls 6mbit, This is a neighboring line that shares the same chorus address That is also just shy of 3 times the speed of mine Despite being far longer.
My pair is near perfect now, having a multiple that was on it dropping a few mbit outright disappearing. (cant see me complaining about that!)
If you would like a quick trial before committing, you could connect the modem up straight at the ETP clean line would be best case situation. - I wouldn't say a master filter itself is much of a cost though, think i have a couple sitting around here now.