Fibre connection is 80/20, achieve this speed wired to the 2830 no problem

2830 on wifi - reaches 40/20 maximum, tried all combinations of settings/security

Just tried another router as an AP plugged into the 2830, achieves 80/20 no problem.

Why is the 2830 wireless slow ?

Will try .rst as a last resort

some has similar problem
http://www.forum.draytek.co.uk/viewtopic.php?f=19&t=19349
but mine as well seems has lower signal compare to other draytek products, how ever the speed for me with adsl is fine (it gets the same as wired) where are coming 4-5 bars to the laptop but i have an adsl line (all wifi settings are with the default settings)
Have you tried latest firmware 3.6.7 with the rst file (it avoids previous firmware conflicts but it sets back to factory default all the settings)
Also if you are not using the second wan disable completely, there is chance for higher speeds if both wan's are enable to stolen some cpu memory so it may prevent rich the full speed

Second wan is off

Tried the .rst, no change

Wifi signal also poor (even with hi-gain aerials) compared to a cheap router I'm now using as an AP.

How far from the 2830 are you getting the speed?
Also have you ever loaded by mistake the firmware from single or dual band models? (wrong firmware), this may cause issues to the wifi driver

when i was beta tester with virgin media testing their new SH2 they do let us use and our own routers
I had speed of 80-90 mbps via wifi out of 100 at 1 and half meter away. that time was with firmware 3.6.3 & 3.6.4, i am not any longer with VM to confirm newer firmwares speed but now with adsl i am getting full speed (that is not much anyway)
If you going to try older firmwares 3.6.3 i remember had issues with android devices when was connected to the wifi (also the rst file will need to use)