There would be no impact on the quality of your line.
The only impact is that with FTTP, there's an ONT, which is a modem. With ADSL/VDSL, the role of the modem is your access point and so by switching it off, you won't probably connect to the internet (because your modem will be switched off). However, it's difficult to tell, as different manufacturers implement different solutions.
I understand that your cabling has been checked etc. but my recommendation of ADSLNation filter comes from the years of experience of being on a noisy line (assuming that this is what has happened in your case). It's a very superior filter and who knows, it may offer that additional stability that you need. You can always send it back.
I'll point out once more that even though the line may have problems, if it's deemed by Openreach to be within the acceptable parameters (this is the phrase they use; never the line is fine etc., but that it's within the acceptable parameters), then there is nothing you can do about it. They'll simply tell you (and at BT too) that a couple of disconnections (can't remember what exactly is the norm now) is fine.
There's also another solution: if you switch providers to [it's similar in name to that that offers road rescue], they have a policy where there's a problem, even long standing, they will fix your line as part of the contract.
