Hi,
Thanks for reaching out to the community.
I am not sure why you want to move to ACS from v7(classic version), ACS is very limited in the case of many functionalities.
I would recommend going for the premium version(ACS+v7) you can get both things in one place anyways let me answer your questions.
1. Is the migration path above correct? If so what is the effort to move from v7 to ACS. I have a pretty good idea of the v6 to v7 migration already (I've done it in the past) but not to ACS:
Yes, you can do that, but you may have to redesign few thing as these two systems are not compatible(Read different systems), ACS is not the next version of v7.
2. Can ACS be used on prem? What about our private cloud?
No, you can't use ACS on Prem.
3. I am aware that the only OS / RDBMS supported today is Debian 7/8 (64bit) for the application service and PostgreSQL 9.3 for the marketing database. Is that still the case?
No, this is not the case, Read this link for Compatibility Matrix
4. We use FDA and the data enrichment activities. Are they currently supported in ACS?
No, FDA is not supported by ACS.
5. We had some pretty impressive email deliver performance times, how does ACS email delivery perform compared to v6?
It's a cloud so you can, make it fast by scaling your network. You will be able to achieve a simmilar response from ACS.
Hope this helps!
Amit