Welcome Atli.
Have you considered using a SQL database with webservices?
I have seen people running into a lot of these usability issues with BDC's. I do not believe they were intended for any heavy applications.
We use SQL, since that was what it was designed for, and web services for complete data integration in InfoPath. You can also send the data back to SQL tables so that you can have aggregation and reporting across the form data.