In theory thin client is a java impl of OCI, in oracle tech doc, they claim 10% faster. You can ignore initial connect time as for sure you will use some kind of connection pooling.
In one of the tax project I involved in the past, they have millions of tax record, I think they always stored it using a single table. If inserting just a one time action?If that's the case, one trade off can be made is use less indexes during heavy data inserting phase; then you can apply indexes back to speed up different queries.
assume you need enter 5 million records in 20 days, 8hr a day, then a given minute rate is >500 record/min. Not a big deal assume you don't have to lock too many other reference resources during a single transaction.