There is no TPS set in stone as with bitcoin, where the blocksize basically determines how many transactions you can fit in a block, with Byteball, theoretically it should handle hundreds of thousands tx/s, but today its not optimized at all for this speed.

I guess the upper practical limit is the 12 witnesses, how many transactions they can see (bot dont need to see it at the same time) and post to each other and append to the database, to the main chain.

There is nothing stopping you from making really a massive amounts of transactions on your own chain - and have it merged to the Main Chain when posting it for the witnesses to see. As long as you didnt double-spend, the verification of which witnesses do takes little time compared to what you spend in making the transactions, if all is fine, your chain of massive amount of txs will be in the main chain.

And none of this needs "proof-of-work".

Isnt this nice? Yes, it is.