We are going to try RDI instead. Our son would have done well regardless of whatever intervention that we picked but we focused on verbal behaviour (ABA) and floortime. Whether son rise or floortime or RDI, ultimately it is the therapist that matters the most. The objective is to engage the child and build relationship. Our ABA therapists were able to do that using a variety of non-ABA techniques. Son rise does have some published research but we wanted a bit more structure (verbal behaviour) if paying therapists whereas we tried to do the unstructured play ourselves.

I've heard good anecdotes but it is very expensive and doesn't have any objective research to back its effectiveness claims. Spending 40 hours/week working 1:1 with the child is likely to be helpful but the question is whether the time and money might be better invested in a different type of intervention like ABA.