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My NA6 is sitting on HSD TT's, with a front strut brace and that's it suspension-wise.
It suffers from horrible bump-steer, the rear end clunks a lot! (I think it's when the springs expand too fast, they hit the perches hard)
The rear makes more noise the higher it is!

It's currently at a legal ride height, and looks crap.
I'm trying to sell it, but I'm pretty indecisive and will probably hold onto it.

If I am to go a little bit more nuts on the suspension, I will probably invest in shortened swaybar end-links for the front (pillowball strut tops) - will this help/cure my bump steer? ( i haven't noticed a change with adjustment of the damping.
I'd also like a diff. traction brace, which I think will help the rear end a bit.

If I do keep it, it will go a fair bit lower, be made a lot lighter, and get a lot more serious.

What will be the best additions to my suspension setup, for shitty street roads?
I want to be able to maintain a visually appealing ride height (not slammed, sensible but low) and corner hard, without jumping all over the place.

Take it to a suspension shop and have them check it over - bump steer is affected by the position of the steering arm in relation to the control arm... see below:

Bump steer has many variables: steering system design, geometric relationship of the steering parts, alignment angles, and sometimes even vehicle maker’s design compromises. All of these variables have the potential to cause bump steer to be severe, minor or possibly not even evident. The primary cause of toe changing under a load is nonparallel steering linkages that cause toe to change in unequal amounts from side-to-side. Nonparallel steering may occur for different reasons, one of them being collision damage.

One situation that causes bump steer is a rack and pinion steering gear not attached parallel to datum. This positions the tie rods at unequal angles from side-to-side. When this occurs and the vehicle travels through a jounce and rebound cycle, the tie rods travel in unequal arcs causing unequal side-to-side lateral movement. Unequal tie rod travel causes a push/pull effect that may be noticeable in the steering wheel. It may also cause the vehicle to have an awkward handling sensation, such as floating, as the vehicle travels through the jounce and rebound cycle.

If a rack and pinion gear is sub-frame (engine cradle) mounted, the same applies. The tie rods must be set at equal angles for both the left and right side of the vehicle. Also, the tie rods must be positioned at near the same angle to both the left and right control arms. Following repairs to a collision damaged vehicle, it’s critical that steering parts are returned to the original geometric position that the vehicle maker designed.

In your case it sounds like something is poorly set up - could even be poor toe settings front and rear. Short sway bar links are only needed on fairly low suspension, and pillow ball mounts should not have any affect on the clunking noise you are hearing, so both would probably be a waste of money in your case.

Mmm okay. I was present when the alignment was done at one of my mates tyre shops. He asked if I wanted extra toe or camber for sliding, i said no, just grip.
The clunking is definitely rear end. I installed it all myself. Everything is tightened up properly and nothing comes loose ever.
My next thought is maybe it's the bushes as well?
When I jack the car up (both sides/front or rear) the control arms are still angled upwards, depending on my current ride height (if it's low, i guess that the bush has rotated about 180˚ from it's stock height, which makes it sit in that position)

The clunking is ridiculous. If I give the steering a bit of a fishtail jerk (go from right to left to right sharply, it clunks, even if it's on smooth tarmac with no bumps whatsoever.)
It's annoying!

Have a look at the steering lash adjustment too, my car was tramlining everywhere, and the steering was vague, with plenty of play. 10 minutes adjusting the lash (plus another 30 minutes grinding the adjustment wheel on a shifter so it would open large enough) and it was transformed

Is the steering rack bolted up properly? Jack the car up and move the front wheels from lock to lock and look for movement in the joints. It could be many things or a combination of things, that's why I recommended you take it to a specialist who can get the car up in the air to check everything.

I doubt you have a bump steer issue, more likely it's the lack of power steering you're feeling, causing kickback through the wheel.

NA6's suffer from a lack of subframe bracing, and your bushings are probably knackered.

Sway bar links won't do anything for what you want.