There's no fast rule about how much speed can be regained by the use of vectoring, but all I do know is that crosstalk has affected my line by 30Mbps so there are some big improvements that it could make.
FWIW, I have gradually lost around 10Mbps or so DS since May 2012 on my 1100m line (from 30+Mbps to 20+Mbps) & around 2 Mbps US, but in percentage terms, that equates to around 33% loss.
That 10Mbps or so loss of speed is very noticeable here & anything that vectoring can do to improve matters would be most welcome indeed.
I'm on a legacy PlusNet product (40/10) & if I didn't need the US speed for transferring files over VPN when I work from home, I'd revert to the 40/2 entry level product that new users have to accept.
The reason for that is that when testing with PlusNet a few years ago, it was confirmed that lower US sync speeds (i.e. capped at 2Mbps) always resulted in higher DS sync speeds on my sub-40Mbps connection & vice-versa.
I more recently experimented by capping US sync speed directly via my HG612 modem, but that didn't have the same effect.
It also
appears that removal of G.INP on US on my connection had a slightly negative effect on DS sync speed, but I don't have any other empirical evidence to state that as fact.
It could just be coincidental with a further increase of crosstalk.