I believe there are sprint cars using A series engines that rev to 10,000 RPM but I dont know anything about them, I can only guess that it would be both difficult and expensive to achieve.

I would think that such an engine from a sprint car would be unsuitable for use in a road car.

The modifications you are talking about would probably be good for in excess of 200HP at the crank in an A15 and somewhat less in an A12.