Without SRA the SNR margin fluctuates with changes in noise whilst the sync stays the same, with SRA the sync speed fluctuates whilst the margin stays the same, but anything so bad that can knock out the connection completely, still does.
Its going to depend on the line.
When I was on adsl, my line suffered from sudden large drops on snr. Even with interleaving it would usually knock out the line, if the line wasnt knocked out, the sync would be generating errors in very high numbers and the connection would be not in a normal operating state (high packet loss). This happened every office working day all year round, the only breaks were weekends, bank holidays and christmas.
I joined ukonline and after a while I managed to get SRA enabled on the line, the benefit was immediate and long lasting. I had gone from dropping sync every day (either manually or by the line been knocked out) to able to hold a sync for over a year. When the noise bursts started, there would still be temporary packet loss before SRA kicked in, but it was for a few seconds only.
So from my experience I would say the prime situation for SRA to provide a benefit, is when a line receives sudden bursts of noise unpredictably that bitswapping cannot handle.
Of course its true SRA cannot increase sync speed over its initial sync, but thats fine, think of the alternatives. Either you resync during a noise burst and maybe hold the sync for a while but forever at the low speed event during periods of no noise, or you just keep dropping the sync every time the noise comes and goes. With SRA you would sync up at the optimal time, highest possible sync, it would automatically lower the sync speed when the burst starts with a few seconds of packet loss. Then the sync would automatically recover when the noise burst ends.
Now with my line it was so bad that without SRA if I synced up during a noise burst, I would not be able to hold the sync 24/7, because my noise bursts only ever happened during office hours in the daytime, and a day time sync would drop out at night because the high tones would get wiped out, syncing the line at night would load up the tones that get wiped out by the noise bursts, so in other words without SRA my line couldnt stay stable 24/7. Even on BT wholesale's most aggressive profile 15db noise margin with interleaving it couldnt hold out. Probably one of the worst lines on the openreach network and SRA dealt with it.
When ukonline closed up, the shock of losing SRA was just too much for the useability and I moved to a congested virgin media service. Didnt come back to dsl until VDSL was available. Luckily for me the problem was E side so now on VDSL I dont get the bursts anymore.
