If you wait for 6 confirmations on a 10 min block chain then with equivalent hashpower you should wait 24 blocks on a 2.5 min block chain. If you are willing to accept 4 confirmations on a 2.5 minute blockchain then 1 confirmation on a 10 minute blockchain provides equivalent security.
This was debunked some time ago. Unfortunately, I don't have a link, but someone made a paper about it. Basically, the more blocks that are produced, regardless of how long it takes, the less volatility there is in the apparent hash power of the network. In short, someone with 1% of the hash power could quite easily create 1 block faster than the rest of the network, and maybe even 2 if they're lucky. 6 is much harder.