Hi Baz, seen other threads that Type R STI indeed is not lowered, or lowered by a minimal amount, by Eibach as it is already lower than normal WRX etc.
Tein is indeed a bit hard.
Comes down to what you want from the car. If daily driver it would be hard to beat Prodrive setup I think and lowering needs to be carefully considered as depends what type of roads and driving you are most likely to encounter. I am in Wicklow and with the bad roads lowering is a bad thing as I need the extra clearance. If going lower maybe 25mm front and 30mm on the back as max values as lowering doesn't just give clearance issues, but also creates other problems with overall geometry which you than need to address by for example getting a roll centre / bumpsteer kit. Those values were for a WRX but if yours is an STI which is already lower than a WRX then you may need subtract maybe 10mm or half those values or you will end up with a car that doesn't handle on anything but flat roads. You will never see a slammed down rally car in Ireland as it just doesn't work on our roads.
FYI, Coil-overs can be every bit as good and comfortable as normal struts. The problem is that a lot of kits are pre spec'ed with springs and dampers and typically they tend to be hard and more track orientated as a lot of people only go through the expense of getting coil over kits if they are going to the track. However, you can order different springs (different spring rates) and even different valving for the shocks with a lot of the coil over providers if you know what you are doing and what to ask for. Have a search on forums to get a better idea of spring rates and valving or speak to an experienced shop if you are tempted to go coil over.
ALK is great and really transformed my STI v3
Polybushing is great, even on a daily driver and if you have any concerns just pick a slightly lower Duro rated compound. Really transformed my STI v3
You could go with a stiffer ARB, however, I found that buying decent WL droplinks was maybe as good an upgrade. Get the big fat C or U shaped ones for the back as the round wire ones are not much better than OEM. The big advantage of the better droplinks is that they work quicker and hence more consistently; from initial shallow turn in there's immediate push on the ARB whereas with the OEM droplinks there is a "delay" for the droplinks to start to load up the ARB.
Another big improvement on the classic is a lower front brace and really tightens things.
With regards your car feeling twitchy: what is the current geo setup? If you are running toe out it will do this. Before you make up your mind on different value you might need, get a geo check to see what values you are currently running.
Hope above helps.