The impact of changing to a long-arm set-up is very small. As the front of the car rolls less than 2mm, the camber change is very small.
Putting a spacer between the upper wishbone mount and the mounting block will give a long-arm set-up and less camber change. Putting a spacer between the mounting block and the lower wishbone will widen the the front width. As H I says...
The bigger change in making the top wishbone 'long' is to lower the front roll centre. That reduces the tendency to grip roll, and reduces the 'bite' of the steering on high-grip tracks. I suspect that the change to roll centre is why this is done, and not so much the change to camber gain.
The lower roll centred will make the car softer in roll, and is usually accompanied by use of softer rear springs. Again, contrary to the usual thinking in On-Road, as the grip goes up making a 12th car softer helps.
Remember that 12th is the only car in the world with a hinge in the middle of the chassis. That means that a lot of the accepted wisdom of race-car set-up doesn't apply. When the main chassis rolls, it uses the centre spring to remain level. As the car turns, the front axle is levered around the centre pivot, the centre shock and the front springs - it's complicated! My experience is that roll centres and weight transfer (position of the rear pivot relative to the rear axle) play a bigger part in getting a car to handle than camber gain. HTH