"In the time it takes you to read this text, 152 passers-by could have seen your ad."
It's conditional. "They could have seen your ad, if you used my services." It's only past in the sense that they've lost time by not subscribing to your service. The message is that they don't have to continue to lose potential customers.
"In the time it takes you to read this text, 152 passers-by could see your ad." This is present tense, but I don't think it's an improvement.
"In the time it takes you to read this text, 152 passers-by will be able to see your ad." This is future. I don't think it has the motivating force of the past tense. How do you know what will happen in the future? The customer is more likely to believe that you know the past. You know that, with your service, 152 people could have seen the ad, and didn't. The customer can work out the odds for the future.
In summary, I think the past tense/conditional version is the best.