Is it computationally feasible to find two transaction IDs with the same first 20 bytes?
Is it computationally feasible to find two transaction IDs with the same first 20 bytes?


20 bytes is still 160 bits of security, which is considered very secure. If you are sure that you only need the IDs to check for existence, 20 bytes of the hashes should be fine. 

Hard drive space is pretty cheap, though, and you might need the full TXID someday just for interoperability between other systems. So make sure 20 bytes really does satisfy your use cases. A 20 byte hash is sufficiently secure, though.