I built up a single OpenLDAP box, just using the standard mdb backend and a ton of RAM. Easily crushed the load test. Sadly could't be implemented for political reasons.
Screw Oracle, especially if you care about performance.
The only scalable LDAP server is the one from Sun Microsystem (openDS). It was killed when Oracle acquired Sun because they already had identity products. Then the sun guys quit to continue the products line under their own company, ForgeRock. https://forgerock.org/opendj/
I've done load testing and this ldap is taking millions of accounts, no problem.
The best products are rarely free once you get out of the average web dev context. Funny thing, this one is both paid and open source.
I'm sorry but there isn't a commercial product that can compete with OpenLDAP and there may never be because it is just that good if you know how to use it. (yes, the learning curve is high, that is the only criticism I really have of OpenLDAP).
The open-source edition is only a facade with highly stripped features.
If you want any decent replication, HA, performances or bugfix. You have to pay for their OpenLDAP gold edition, which last I checked was $100k for a site license.
If you don't pay Symas money then no, you won't get support, or bugfixes developed for you.
However I doubt there is a large performance difference, it's already very efficient so it would be in the single percentage points.
As for replication you can do almost anything with the open-source version by combining syncrepl/delta-syncrepl into the topology you want, hell you can do N-way multi-master if you wanted.
I'm not sure what you have against using open-source OpenLDAP, maybe you should try it first?
Technically it works... sometimes. Custom shims have a tendency to lead to complications because the API isn't very well-exposed or documented... it was another acquisition product.
Infrastructurally it's an unmitigated garbage fire. Oracle seems to think, e.g., your organization is too stupid (which is true, since you're an Oracle client) to know how to connect to AD from a nix machine or Java application, so the hardware footprint is huge.
It's questionable biz-wise for two reasons. First, because it's sold as a "complete", off-the-shelf solution when in reality it's probably about a dozen analysts working on it full-time for months just to get its biz logic set up, and then a handful for maintenance. Second, because OIM aims to be a institution-wide, one-stop-shop product (which it is* good at), but institutions that acquire it are so large that they're bereft with inter-department politics and policies that make using it firm-wide difficult.
As a matter of fact, I did so multiple times in order to provide simple JSON APIs which consume data from massively huge enterprise software and expose them to nice shiny web apps. Can't name any more details though, but it works - at least if you use PHP, but I believe there's also an Oracle DB driver for nodejs, and most certainly for Java/JDBC.
Everyone who needs Oracle deserves Oracle.