Throughput testing has focused solely on native (NEAR) transfers, not any smart contract execution. With 8 shards we have observed almost 4000 tps working well, where the transactions in these experiments were only NEAR transfers.
Testing the throughput of smart contract execution will certainly be done in the future. Using NEP-21 as one of the benchmark contracts is an excellent idea! Are there other contracts you would be interested in seeing tps measurements for?