So after two weeks of monitoring I can tell you, that RCSI had no significant performance impact on my system.

I even had the situation that the job importing was overlapping with the aggregation job (due to late delivery of the source system) and the import was not locked by the aggregation.

I also checked that the results of the aggregation where calculated correctly and it behaved like supposed to: it just used the data that have been there at the beginn of the query.

Thanks again for your advice

Christian