Lets start with a few (short) answers:
- Q: is splitting the DXA Master publication supported
- A: in short: no. slightly longer: depending on what you expect under supported, but as you already mentioned, if you split it up by doing partial imports, future upgrade import scripts won't know where your items are and might import them in the wrong location.
- Q: are there any other reasons not to do this
- A: nothing other than because you don't need to (I'll come back on that)
- Q: could this be a future enhancement/option to support this
- A: don't count on it, because of the issues I explained in your first question and because of why you shouldn't do it (see below).
So lets look at why you exactly would want to do it. You mention, because you have a Publication split for Schemas and Templates already.
That Publication split was created because we were incorporating design in the Templates, and wanted to be able to split separate designs (note: in the original BluePrint diamond model, it was called a Design publication, not a Templates Publication). With DXA your implementation changed, not only on the CD side, but also on the CM side. Design is no longer stored in Templates, and in fact Templates themselves have (almost) become obsolete. Templates are just a configuration item in DXA with relevant metadata, they basically all contain the same TBBs inside. Hence we dropped the Design Publication from the BluePrint design.
So then to what I think you should consider doing. If you currently still have a need for your Design Publication (because you are using (DWT) Templates with design elements in them, keep it there (else consider removing it, because it has no further use). Then where should you import the DXA Master Publication into, in your 100 Schema Master Publication.
Following up on @dylan-mark-saunders comment:
There is an issue with adding the DXA Templates into the Master Publication when you still have traditionally Templated sites. Namely that the DXA templates will now be exposed in every publication below it, and can thus accidentally be selected.
There are a couple of possible options to prevent that:
- through security (which might be impossible if you have the same people dealing with both sites)
- after import, use the BluePrint move-down feature of Web 8 (but then you will have to keep doing this with every update of DXA unfortunately)
- import the entire DXA Master Publication as a separate Design Publication
Number 3 has the downside that you might end up with duplicated Schemas, although I guess you always have that issue when you import DXA with the core module in an existing Publication.