I am familiar with phenolic resin impregnated carbon tubes used in services like HCl cooler absorbers and some extremely thin coatings that have been applied to the outside of economizer tubes, but I do not think I am familiar with the tubes you speak of.  The reduction in heat transfer performance can be calculated by knowing the thickness and thermal conductivity of the phenolic resin coating.  In applications where the overall heat transfer coefficient is already quite low, this may not have much of an effect. However, if the overall coefficient is quite high then the effect may be dramatic.  For a fouling system, one must evaluate the reduction in clean heat transfer performance against the mean time between shutdowns for cleaning.