Lyle Overbay vs RHP: .296/.339/.546
Mark Teixiera vs RHP (2012): .239/.331/.438
The biggest upgrade that Tex will provide will be vs LHP. But even that only represents ~30% of the total at bats. Mind you, it's an enormous upgrade (Tex .865 OPS vs LHP in 2012, Overbay .416 OPS vs LHP in 2013) but again, you face a large majority of RHP rather than LHP.
Sidenote: Can we please stop pretending that, just because Tex missed the actual month of April, he won't have a slow start?
He's a historically slow starter (career .238/.342/.423 line in March/April). Not only does he start slow when healthy, but he's now had an injury, and not just any injury, a wrist injury.
Ask David Ortiz how it feels to hit after a wrist injury. Or Will Middlebrooks.