If your application is poorly written (from a modern perspective, this is implicit in that RAC white paper), RAC will make it worse. I would look carefully at something that has been around that long: are there many hints? Is it even using the cost based optimizer? Is it designed relationally for performance? Who wrote the app? With which tools?
There are white papers floating about that tell you what to expect from the optimizer when upgrading from 9 to 10 and when upgrading from 10 to 11, you might look at those and take another look at what your application expects.
Yes, your requirements should control the decision, but your existing application needs to have a say so you can know how to get there. It's entirely possible a jump from 8 to 11 will let the app will work better simply from more modern hardware, but if it is actively counter to basic relational assumptions, it could go haywire.