Before trying anything else - have you got the latest Netgear firmware - there was a recentish release which could be beneficial for long lines.
Has your speed dropped due to an increase in the Target SNR?
The best indication of your target SNR is by looking at your
line stats immediately after a reboot.
Yes, sometimes the routers you have mentioned can help, but in other cases the Netgear still performs well, particularly with the new firmware.
If I sound hesitant its only because I dont want you shelling out for a new router which may or may not help, if theres something else that can be tried first.