You just need to visualize really how "long term" the investment is.
If you have a singleplayer world, and you play three hours a day, every day, then you will be playing for an entire
month before a 36HP boiler will be at 90% overall. And then you're still not getting the advertised 290,909 MJ per bucket of biofuel, but rather an average 261,818, which is almost 30,000 less. Chances are that during that month, FTB updated the modpack twice and most people will have started at least one new world. So you actually have a funny situation where a world gets retired before the boiler gets anywhere close to maximum efficiency
Of course, SMP servers suffer less from that, although they face different problems... actually using the almost 250 million MJ per day that will be produced per 36HP boiler without wasting any while the owner is offline, for example. And the 856 buckets of biofuel consumption per day aren't exactly negligible either.
Boilers of this size are really only valid if you have an infinite fuel loop set up, and then, efficiency becomes completely meaningless and only output counts. It gets far more interesting in a limited resource environment, for example with "hard mode" config settings that nerf a lot of the common "spend 20 minutes building, have infinite energy forever" kind of loops. When in such a situation, choosing the proper boiler size for the amount of fuel you have stocked can be very rewarding, and that's what my charts are going to be about.