I would suggest that there _are_ quite a few things that are worth trying.
I myself have a similar very long line. I have put a lot of work into testing noise-lowering measures recently with good results.
A few points,
i) Is the -GT a Broadcom-based device?
ii) consider non-wireless routers (and simply pick up a better access point separately - I recommend the Netgear WN820T or better the newer 5GHz device [yet to be tested[) in case that allows you to get a quieter noise environment
ii) consider the DG834v3 (non-wireless) as I get the impression that this out-performs the DG834v4 which is Broadcom-based on very long lines, but this is yet to be confirmed as I need to do much more testing.
iii) the 63.5dB line attenuation reading is what I get too from a DG834v4; the figure varies between router models DG834v1 reports 59dB, v3=61dB, v4=63.5dB. If this is really the picture that the v4 Broadcom device 'sees' then it must be really suffering. Or maybe it's just an artifact of the way it calculates line attn.
iv) experiments suggest that it is well worth implementing a range of RFI and noise reducing measures; and of course you have got rid of all your extensions and had a faceplate-type microfilter fitted to your now one-and-only NTE5 master socket?
v) compare the Adsl Nation XTE-2005 with the 'Pressac' NTE-2005 as I'm of the opinion that sometimes the latter may outperform the former.
vi) use short wallsocket-to-router cables, which IMO should possibly be of the UNshielded twisted pair RJ11-RJ11 as my experience has suggested that the foil shielded type can sometimes be slightly worse, possibly increases attenuation? capacitance? (And what about the lack of grounding of the shield?
vii) Particularly if you have a cordless, phone retest _during a call_; my DECT phone worsened SNR by 2dB but only when a call was being made (compared with turning it off/unpluging it); the phone cable seemed to be acting as a transmitting aerial and so changing the cable to a shielded RJ11-to-BT cable helped enormously, possibly using a ferrite coke in it would do too, probably should put a ferrite on its power cable as well.
I did all of the above (and a lot of other anti-RFI and mains noise measures too) and managed to just about get from the 1500 IP profile sync range up to just about scraping a sync rate of 2272 at 5.5dB-ish which was held up for four days. Anyway, the sync range for the 1750 IP profile should be easily achievable even with the deterioration in the weather.