I believe it does make change in some cases, read under PPPoE which vdls is using, not for the speed but for the unstable line drops over time
http://aa.net.uk/kb-broadband-mtu.html

https://community.bt.com/t5/ADSL-Copper-Broadband-Speed/BT-INFINITY-HUB2/m-p/29856/highlight/true#M18519
http://forums.thinkbroadband.com/btsupplier/3820290-the-bt-fttc-line-stats-thread.html?fpart=all

I think we are talking about differenct things.

I am not talking about transfer rate where MTU becomes important. I am talking about the sync rate between the modem and the DSLAM, this is before any packets are sent.

If i understand correct using the 2860 as direct modem, it does connect fine but the speed get lower over time (from 62mbps down to 50)
If this is correct then the DLM must find an unstable line (noise) due to the snr & errors (even the 2860 reporting 0 must bug)
i am not saying MTU (transfer rate) is responsible for synchronization but for your issue (together with snr / errors over time) may play role, many believe 1492 is the best for FTTC

The modem code on drayteks are responsible for synchronization & for stability
http://www.draytek.com/index.php?option=com_k2&view=item&id=1226&Itemid=293&lang=en

Apart the MTU theory (which i may be wrong) I also believe if you try sometime & the VECTOR firmware & the other alternative modem code build in, may improve the line drops

if you type via telnet
vdsl ?
what options are giving you? because there are some bit swap , snr, etc which may enable (if the commands working-bug issue) & helps keep the line stable so less synchronization drops

Here's a telnet session to show what's available. I've missed a couple out as I cannot post more than 3000 chars.

At present I am running EWAN so VDSL is note connected, however you can see what's available.

> vdsl help
% Valid subcommands are:
status idle drivermode reboot annex optn
savecfg vendorid snr erb

> vdsl status

---

ATU-R Info (hw: annex A, f/w: annex X)

---

Running Mode : State :
DS Actual Rate : 0 bps US Actual Rate : 0 bps
DS Attainable Rate : 0 bps US Attainable Rate : 0 bps
DS Path Mode : Fast US Path Mode : Fast
DS Interleave Depth : 0 US Interleave Depth : 0
NE Current Attenuation : 0 dB Cur SNR Margin : 0 dB
DS actual PSD : 0. 0 dB US actual PSD : 0. 0 dB
NE CRC Count : 0 FE CRC Count : 0
NE ES Count : 0 FE ES Count : 0
Xdsl Reset Times : 0 Xdsl Link Times : 0
ITU Version[0] : b5004946 ITU Version[1] : 544e0000
ADSL Firmware Version :
Power Management Mode : DSL_G997_PMS_NA

---

ATU-C Info

---

Far Current Attenuation : 0 dB Far SNR Margin : 0 dB
CO ITU Version[0] : 00000000 CO ITU Version[1] : 00000000
DSLAM CHIPSET VENDOR : < unknown >

> vdsl annex
% hardware is annex A.
% VDSL2 modem code is annex A/B/C
>
> vdsl optn
%% adsl optn FUNC [us/ds/bi [value/on/off]]
%% FUNC : 'trellis', 'bitswap', 'sra', 'retx', 'aelem', 'status', 'g.vector', 'default'.
%% 'default' : set all feature back to default.
%% us/ds/bi : upstream or downstream or bidirection.
%% 'aelem' is only on/off.
%% value : bitswap=0~2, sra=0~4.
%% on/off : enable/disable.
%%
%% PS, some commands are available only for VDSL2.
%% For G.Vector feature, CPE's modem firmware needs to support VDSL2 G.Vector.
>
>
> vdsl snr
Usage:
adsl snr [delta]
delta: SNR margin delta. The SNR margin delta should range from -50 to 50.
Current ADSL SNR Margin is 0 dB.
>
> vdsl erb
% vdsl erb [ tag | untag | status | txdiag | rxdiag ] [ on | off ]
% tag: ERB tagged with vlan ID defined in 'wan phyvlan wan 1'
% untag: Untagged ERB packet
% txdiag: Diagnostic log for egress ERB packet
% rxdiag: Diagnostic log for ingress ERB packet
% on: Enable diagnostic
% off: Disable diagnostic
% Current status of ERB tagging: UNTAGGED
% Current status of txdiag: ON and rxdiag: ON
>

Thanks ... i am not sure if the commands will work (due to bug because all meters are to 0) but you try the following commands which are responsible for better stable line if will help

vdsl opt bitswap US 1

vdsl opt bitswap DS 1

(that should enable bit swap at higher level)

vdsl opt sra 4 (i think is 4) can try 3 as well

(this is dynamic sos, should help minimize any line noise)

If you have the vector version can try as well

vdsl opt g.vector on or enable

if not works try help in each submenu, it should show you available commands

If are accepting the commands but not any change in values can be a bug in the commands ... can replace the command with adsl but not sure if will work with vdsl

If will accept the commands then the

vdsl savecfg

should keep in memory after each reboot of the modem

This vectoring fimware thats been mentioned for the 2860, where can it be downloaded? I'm currently on what I think is the latest Firmware Version 3.7.4.3 Build Date/Time Sep 15 2014 17:33:55
DSL Version 548006_A