Enlarging the strut mount slots (adding camber)
 

I'm guessing I can add some negative camber by enlarging the slots in strut tower. They are already slotted so I don't really see the downside. Does anyone know how much negative camber you can get with this method?

Total camber will depend on several factors including ride height and any modifications you may have done to your suspension - such as coilovers etc. I have -3 degrees in the front with stock control arms and slotted towers but the front is pretty damn low with shorter springs on Moton coilovers.

Downside?

Is this a race car or a street car?

If its a street car, then consider when you try to sell the car and the shop inspecting the car tells the prospective buyer that the shock towers have been permanently modified and there is no way to return to car to stock condition?

The prospective owner then asks the shop why someone would perform this type of modification and the shop says "most likely to get more neg camber so they could track the car". You say that it was never tracked but the buyer walks away anyway as there is no reason to take the risk on your car with so many other un-modified cars for sale.

Very few buyers will trust that an owner knew what he was doing when modifying the shock towers and will assume the worst (that the car will suffer from long-term alignment issues that can never be repaired).

I would suggest camber plates or adjustable lower control arms which can be advertised as a upgrade and/or which can be removed for re-sale without permanent damage to the car.

I've done this on both my Boxsters. The amount you grind out is maybe 5mm or maybe a bit more. If you are near and tidy you can barely tell the difference. At the end when finished there is a plastic cap that covers this area and then a scuttle cover. I would be surprised if any service tech even noticed. I run approx -2 all round on a road only car.

It will also most likely take you out of any street stock class and would force you to run with modified cars. If that was your only performance mod, you would probably not be competitive

Do you know the size of the cutting Drill bit you used off hand?

It's just a grinding type tool on the end of a dremel, even better if you have an air tool. Don't overthink it. Whatever fits in the slot just push in the direction you want to grind out, the existing slot acts as your guide. Use a felt pen to mark where you want to grind to. Dust down the area then use some touch up lacquer or colour or similar on the exposed metal

That would definitly work. A cutting drill would allow you to achieve a pretty factory look.

Is there any reason for the resistance to using camber plates?

FWIW, -2 deg camber on a street driven car will certainly help in the corners but unless you drive continuously very fast in circles, the tires will experience a very high wear rate on the inside portion of the tire. This wear can easily be missed unless the entire width of the tire (and especially the inside portion) is inspected regularly. Many owners not used to this level of camber and uneven wear are shocked when the inside is worn down to the cord and there is still 70% new tire on the outside (easily visible) portion of the tire.

I've done 5000 or so pleasure miles with this set up and the tyre wear is even enough across the full tread. 3000 of this is pretty hard driving in just 2 weeks. Of course that's not to say it works for everybody. I wasn't recommending any particular geo though. I'm also in the UK although more than half those miles are in continental Europe not that it makes that much difference as such. Mines a fun car solely for fast and fun driving.