Lots of guys here with much more knowledge than me, but the just of it is this: First, I think most people who take the time and effort to port intakes (and intake manifolds), usually port exhausts also. Second, the exhaust has the benefit of the piston forcing out the spent combustion gasses, so free flow isn't as critical to power production. Third, I think it's harder to make significant gains in the exhaust port because of it's short runner length, extreme angle of exit etc. Fourth, there is only so much you can benefit from on the exhaust side when many FE cars have shock towers that require the exhaust to abruptly turn immediately after the port exit...negating much of the gains that could benefit from a free flowing exhaust port.
Having said that though, there is a recommended percentage ratio target for exhaust to intake flow. Trying to achieve that target ratio is a good thing, if you have the money and/or means to do it.