Kev
I've had similar problems with my mk4 Golf in that repeated alignments failed to sort out the suspension problems. If you make the basic assumption that the guys doing the alignment know what they're about then the conclusion that I came to was that, since the alignment is effectively a static test, then the suspension is moving about under load. You don't mention what mileage you've done but in my case most of the suspension components were completely knackered by about 60k miles. These things are built down to a price and they just don't last.
What have you replaced so far? For me I've done front wishbones, top mounts, bearings, anti-roll bar bushes, drop links and all four dampers / struts. It's taken all of that, along with 5 alignments, to get it to drive straight without any odd tyre wear. The rubber bushes on the rear beam will need doing soon but that's a painful job so I've been putting it off (there's too much toe-in at the rear but at least it's even now).
There isn't much alignment you can do on a Golf. You can do the tracking at the front and there's a tiny amount of camber adjustment where the ball joints bolt on but, other than that, I think you're into moving subframes to even things out left to right or adding shims. Personally, at the age it must be, I'd plump for most of the suspension being well past its best and overdue for replacement.
Impreza's have a fundamentally different 4wd system to the haldex system in a Golf, by the way. The Golf system isn't full time 4wd as I understand it - it's 2wd for most of the time.
Regards
Duncan