I did about 10 months of ABA with ds. He progressed hugely. I had a year of not doing ABA. He stayed where he was (which was remarkable, and beyond my expectations but not good enough). We have started ABA again and he is flying once more.
My ds is HFA, I think. He was dx as moderate-severe. Perhaps he is both. Indpendent EP thinks he might be aspergers in a year or two.
The point I am trying to make is that where the child is on the spectrum is a bit irrelevant, particularly towards the HFA end.
BUT, the 'type' of ABA is critical and unfortunately the more HF your child the less expertise there is out there. It is one thing to teach a child their colours, but quite another to teach them social interaction.
Also, when you are starting out yourself learning the principles of ABA it is much easier to understand and practise when applied to teaching colours than social skills.
Also, even if you have ABA understanding and principles which if applied to social skills WILL help, conjuring up the social situations and peers in order to practise (ABA is fundamentally about creating loads of opportunities to practise) can be hard. ABA might come with manuals and trained experts, but it doesn't come with a bunch of willing kids.