~ if testnet has a small fraction of mainnet hashrate (for example 1%, or even less than that), then any mining pool, at any time, can launch 51% attack
Testnet's hashrate is closer to 0.0001% of mainnet, right? It's really low.Hey, if you ever want to test how a 51% attack would work, testnet would probably be the chain you'd use to do so 
If I understand correctly, this would be possible with much less than 51%:Quote
Shower thought: I'm not sure how "chain work" competes against "chain length": can 1 difficult ASIC block wipe out thousands of easy CPU blocks?
Yes. The main limitation is difficulty adjustment: you cannot make "stronger" block, than your difficulty says (because then, everyone would just work on top of the Genesis Block, all the time, and would be guaranteed to throw away the whole chain in the future, with constantly increasing probability).