It's not really centralising it, more so decreasing the security of a decentralised network.
As I mentioned in the post, a shard take over is possible, which answers your second question - and yes/no, because while shards are more susceptible to attacks, if they can't actually interact with other shards (instead use receipt tracking), then threats will be easily isolated, so it's a win-lose situation unfortunately. They're working on fixing this.
Yes, test net results have increased TPS by 233% already, with the aim of going even higher, although I don't know the exact number of TPS sharding will allow.
Thanks for the comment!