Do you mean 2.5 WAR is the most likely average of possible outcomes, or you think 2.5 WAR is the most likely outcome? I agree, this guy is an enigma wrapped up in question marks. He could put up a 0 WAR season, a 2.5 WAR season, a 6 WAR season - I feel like none of these outcomes would be a surprise and all are equally possible.
It's hard for me to see him as acceptable gamble at the likely price tag ($160-$200M) and I don't think there's any statistical magic that indicates he's most likely 4-6 WAR player moving forward. I'd prefer they go the route of shorter term signings and save the payroll space for stronger FA classes next year and the year after that. It's just poor salary management to pay for the upside scenario on someone who has such a variability in possible production.
I'm still going to be pumped if we land him, I'll just be crossing my fingers and hoping the dice fall in our favour.