Not anymore. After the initial heat-up, they actually become less efficient the hotter they get (much less for HPs).
No they don't. They get more efficient as heat goes up, just like before.
Whoa, seriously ? WTH is CJ doing with his mod ?
Fixing a gross balance issue on the solid fuel side, buffing liquid fuels back up from Player's changes to maintain their usefulness, making boilers more realistic, making boilers more convenient to use, adding a config option for adjusting boiler base fuel efficiency, removing heatup costs, and changing the way HP fuel inefficiency is handled.
Previously boilers started at 900% fuel consumption and dropped to 100% at full heat, now they start at 100% and increase to 110% at full heat. This means heatup costs as we know them are more or less gone. Instead, steam output scales by temperature (which is the part that Seraph failed to take into account). This behavior is not only more realistic, but also allows you to fire a large boiler up with only a small supply of fuel (or even directly on the output of a fuel producing machine) instead of bunkering chests upon chests of buffer fuel. And you don't lose a gigantic investment in fuel if your boiler accidentally cools down while you're offline.
Previously, the efficiency difference between LP and HP boilers was a short term efficiency penalty on the HP side through quadrupled heatup costs compared to the LP side. Now that heatup costs are largely gone, there needed to be a new way to make HP boilers less efficient than LP boilers (which has always been the name of the game). It was realized by adding an additional fuel cost on top of the double fuel draw for double output, making it a long-term efficiency penalty instead of a short term one.
Previously solid boilers were about 6-8 times as good as solid fuel generators, and 1.5 times as good as liquid fuel generators. Now, they are still 1.5 times as good as liquid fuel generators, but the solid side has come down to a much more reasonable 2-4 times. This was achieved by doubling overall fuel costs but also doubling liquid fuel burn values (turning Player's /4 nerf into a /2 instead), while adjusting some of the more powerful solids downwards.
It was a much needed change and leaves boilers in a better spot balance- and usability-wise. They are still the single most fuel efficient power producers across the entire ecosystem of Buildcraft, Industrialcraft, Thermal Expansion and all its associates/addons, for both solid and liquid fuels.