Oil (or gas) fired hot water heaters exhaust do get very, very hot. Probably not a lot you can do, that is way many are designed, the heat just goes right up the middle,
They do make better fuel efficient ones, where the exhaust is piped up/around the tank to reduce the heat exhaust output and lower the temps and actully just use PVC pipe, but are very, very pricey.
here is one with a cutaway view and you can see how the exhaust extracts as much heat as possible before exitinghttp://www.americanwaterheater.com/prod ... lpg100.pdf
If it really cold outside, you baro may open a bit due to the fast heat rising up the chimney and creating more draft, but most of the time, the baro's, if adjusted correctly, normally remain pretty stationary. All depends on the construction of your chimney.