From my experience, unless the plane has huge ailerons, the effect seen from flapperon/spoileron is very small. A trainer will not have huge ailerons, by huge, I mean funfly/3D types, i.e. full span greater than 15% chord. I don’t know what you mean by “heavily modified advanced trainer”, but I’m guessing small ailerons. While I encourage you to experiment safely, I think you will come to the same conclusion, i.e. not really effective.

But, with big ailerons, such as on funfly/3d types, you can pull off some very interesting flying with huge flapperon/spoileron throws, like on the order of 30-60 degrees of throw. But as others mentioned there are some bad side effects so start small and use a switch to enable/disable the mix incase the mix is really naughty.