No the testnet data will be unstable and likely lost. Testnet3 may be different but testnet2 data will be gone. The serialisation and account mechanisms are being re-factored in parallel with the features like mail etc. being created. So we are balancing these two things, allow devs to get an API that will be stable, but underlying infrastructure changing.
Its what the testnets are really for, it allow us to damage and break the underlying tech and improve it for beta. So we see it works, that is good, but we also get to measure how to improve efficiencies. The current sprint for instance was able to reduce account info to a fraction of the original allowing us to run nodes on much smaller less well connected devices, and improve security at the same time.
So for testnet2 data is deleted for sure, testnet3 may not but we are racing in the background with the wire formats and account info work. The work it saves in the long run is immense so the balance is a fine one, but worth it we think. Even if it puts us under much more strain in the core team.