Hi Mark,
the HEQ5 pro is a fairly solid little mount, i have a FS-102 (820mm focal length) mounted side by side with an ED80 on mine, i haven't re-greased or modified it in any way and it seems pretty solid with what i have on it.
i'm using mine on a portable pier which although much much more convenient doesnt seem *quite* as solid as the tripod
i would almost definately be over the 'max' photographic load but aslong as its balanced well and theres no cables hanging say from the camera to the ground i havent had any real problems taking 15 minute subs aslong as my polar alignment is good
heres a couple recent shots:
http://users.on.net/~monox/NGC2070v3.jpg (12x10min)
http://users.on.net/~monox/M42.jpg (10x10min)
the rig itself:
http://users.on.net/~monox/imagingrig.jpg
also take a look here:
http://ozastro.dyndns.org/index.html
scroll half way down and look for 'My Synta HEQ5 Project'