Yes & no. Firstly, a poor-quality cable can limit speed by causing too many errors and retransmits but the cable itself does not determine the speed. That is determined by the devices at each end. Consequently, a better cable will not get you a better speed as such.
A lot of people would point out that the signal has already travelled a long way to reach you, so extending the cable is a tiny bit further by comparison. That’s a fair point and so opinion varies on this.
That having been said, the cheap RJ11 cables are often based on the old four straight wires that were used for a voice phone. These were fine for voice but they are lousy for the higher data rates. They are prone to interference and cross-talk and so can deteriorate quickly with distance. A lot depends on the amount of interference in the environment you are running them through. Personally, I would say if you want to minimise interference, get a “twisted pair” type cable. Just be aware, a lot of them are overpriced, for what they are. The “Cat” cables in particular are over the top for that type of connection. They are intended for much higher speed connections but if you can get one with the right plugs and at a sensible price it will work fine. (Just be aware that these “Cat” cables are RJ45 plugs with 8 wires. They may not fit the socket).
Distance tends to be the biggest issue in my experience and I would try to keep it to under 5m. Even with good quality cable you may still lose 1 or 2Mb/s
I don’t know whether they still do them, but I have used ADSLNation Pro+ cable in the past with good results, at a sensible price.
