A lot depends on what you are permitting to discharge into your system. The headline "15kV" on serial drivers is usually "Human Body Model" - a 100 pF, charged to 15kV, this is discharged via a 1.5 k resistor.

Now try your serial port with a machine model - 200pF, no limiting resistor, or limited by the series inductance, a few nH, and you get completly different answers. Your CAN driver's 2kV come from a machine model. Your series drivers may still need beefing up because they won't handle the jolt from a 200pF discharge.

Steve