The short length of cable from the modem to filter is insignificant compared to the length of cable to your premises. The only thing is that DSL is a balanced signal and to send it through a non-twisted cable for the last couple of meters is not a great idea where there is usually a lot of noisy electrical equipment near by.
Most people will see little or no improvement in sync speed as it is not as significant as things like positioning and quality of filters. If you already have a Pro RJ11 cable then it makes sense to use it as the twisted pair cable is better at rejecting interference. Better than downgrading so to speak to a non-twisted cable.
The main reason we produced the Pro+ cables was for long runs like 10m where it often does make a difference as cable is running past various electrical devices, but people want short lengths so may as well have a twisted pair cable rather than a poor flat cable.