I agree that you may have heard wrong. At least I hope you did, and that recruiting isn't giving blatantly incorrect information. United didn't hire close to 600 in 2017. However, at this point it's a reasonable estimate for 2018 and beyond, considering their retirements and growth plans. We'll see. If that ends up being true, then 150 per year from the XJT CPP isn't bad.
Upgrade times will likely drop. They *could* possibly go down to 3 years for someone hired now, but that's a big unknown. Transitioning ASA pilots will be filling captain vacancies for a while, so that could cause the upgrade times to stagnate before going down again. Either way, I wouldn't come to ExpressJet if upgrade time is most important to you.
That said, the potential time it would take for a new hire to get to UAL through the CPP is fast, and upgrading is not required. I would say it's likely even less than 5 years, which should be faster than any of the flow programs at other regionals. As you mentioned, you have to be successful with the Hogan, interview, and possess a 4 year degree. These hurdles catch a lot of people, but the result is a shorter line for those who make it through.