Well, you wouldn't just lower it significantly and then hope the suspension geometry has remained somehow unchanged- would you?
A full lazer geometry set-up is required and where necessary suspension parts may need to be substituted for adjustable upgrades as you suggest.

Tyre pressures can also cause this, with track use you need to drop the pressures before hand!

Depends on how much inside tyre wear you can live with.

My road car is only at about -1.5 degrees and that probably wears the inside edge by about 2mm more than the outside edge over the course of it's life. i.e. when the inside is needing replacement, the outside still has 3.5-4mm.

If you add further negative camber then you'll get even more uneven wear.

My answer would be either to spend the money and get it appropriately set up with whatever parts are required by a shop that knows their E36 track cars or to reverse whatever you did to lower it. How much camber to run depends on the tyres, pressures, driving style and suspension setup so there is no simple answer to that question. The baseline is that you need enough camber to not roll over the shoulders of the tyre and probably not any more if you are running standard road rubber. It is equally possible that the answer might be that you need to drive faster

I'm picturing a barried up E36 .

Seriously though, a good tyre fitter will be able to tell you your current toe, camber, and the rest. You can go from there to getting it back near to factory settings. Or, say, find out what the M3 has and get it set the same. How low is this thing? Pics!

Ooook that's very nice

In answer to your question, Camber and to a lesser extent Toe angles effect handling and tyre wear and once appropriately set-up your car will handle better and use it's tyres properly. Perhaps someone in one of the BMW racing championships can furnish you with some recommendations for these values as a starting point?

There is a limited amount of camber adjustment at the rear via an eccentric bolt on the lower arm. This will probably be seized solid unless someone has dealt with it already.

After that it is adjustable camber arms (cheap and cheerful steel ones on ebay or nicer alloy ones from the likes of Eibach cost a lot more)

Check the toe

This.

It was toe out that did similar to front tyres on my car (outside barely worn, inside down to carcass after 5000 miles), not the camber.

Get an alignment/geo done now, otherwise, you'll spend more than that on replacement tyres.

On my car it was caused by the previous owner replacing wishbones or track rods by himself and not getting an alignment done afterwards.

No, it's very well balanced in the dry.

Understeers in the wey in slow corners and oversteers in fast corners, but i'd expect that from a RWD car.

Is the rear toe even adjustable on these?