According to Internet Retailer you conversion rate will go up by 400% if you can use personas, possibly in your PPC Advertising - and elsewhere.  Like everything else - the results depend on the implementation:

"…When it comes to personas, the return you get depends on how much you're able to implement."

And…perhaps the best reason to use personas is to understand your audience - before you even think about converting them.

To me, the talk about Persona's from Grokdotcom (where i first read about this story) reminds me of the Systems Admin work I used to do in data centers, in another life before this one, where successful large implementations were based on figuring out a successful architecture for data-centers all over the world.

In other words, if you got the server architecture right - or in our case, the personas right, everything else could be layered on top of it - including keywords for you PPC campaign, the SEO work, Navigation, even creative.

The problem, of course, is in the implementation; sometimes, by their nature, companies can't really implement it due to internal roadblocks, politics and lack of trust for the outside vendor.

So, my feeling is personas will increase your conversion rate by 400% to the extent your willing to work with a client firm that does this work and have chosen the right one.