We would clearly need to do a deep dive but since Orly was designed with modules in mind it should be feasible to use the storage engine underneath Cayley.
This is one place where Log-Structured-Merge systems can really prove themselves. There is rarely a need to sync on every change as most of them are independent. You can usually gain lots of write-throughput by syncing at the speed the hardware is optimized for and squeezing as many append-only changes in to those logs. At Orly we've spent quite some time looking at ways to deal with these bottle-necks.
I agree with the idea that tables are just strict graphs and as such a graph database is usually capable of substituting a relational database. I think many graph DBs lack a sophisticated enough query language to bridge that gap. At Orly (https://github.com/orlyatomics/orly) we're working on a powerful query language, and it's nice to see that Cayley is doing the same.
> querying from two different databases is going to slow down my responses
I think querying 2 different systems tends to be slower, but more importantly you lose transactionality. If you can use a single system that is at least on-par with your relational system for your run of the mill data and have a very powerful graph then that's a big win.