Why would you need to "catch up" in the 51% attack?
You're exactly right; there's no need to catch up if you don't start off behind.

As soon as I get 51% of the network's hashing power, I start building my fork in private, then buy something on the 'real' fork and allow the purchase to get as many confirmations as it needs.  It will be a lot, because I'll be buying something really expensive.  I take care not to include my transaction in the private fork.

Then, once the goods are shipped I publish my secret fork and get my coins back.

The Finney attack is only good for double spending when the recipient accepts unconfirmed transactions.