It is possible that other mods could change these values, but...
All boilers produce the same amount of steam, higher tiers just produce HOTTER steam.
Due to the way steam engines work, you need to use a boiler with the same tier steam engine. Using a lower tier steam engine results in loss of fuel, as the steam engine will just use the steam up to the value it is rated for, wasting excess energy. Using a higher tier steam engine doesn't lose anything though.
So you should upgrade the steam engine first, then the boiler.
Also keep in mind that Fluid powered boilers are offset by a level, so a MK1 Fluid Burning Boiler produces the same temperature of steam as a MK2 Boiler. It also produces an extra half of the quantity of steam, so a MK1 fluid burning boiler can power 3 MK2 steam engines, where the standard boiler can only power 2.
Keeping with that, the Heat Exchanger is then offset by 2 tiers, so the MK1 Heat Exchanger produces the same temperature steam as the MK3 boiler. Except it produces double the amount of steam, so the heat exchanger can power 4 MK3 steam engines.
This is also the tier steam turbines become available, and a Steam Turbine is worth 2 Steam engines. So at this level, a MK3 boiler can power 1 Steam Turbine, a MK2 Fluid Burning Boiler can power 1.5 Steam Turbines, or a MK1 Heat Exchanger can power 2 Steam Turbines.
These ratios continue up the scale.
When you do get this high, even if you're not planning to use Nuclear, you should look into the Heat Exchangers, power them from Heat Sources (Burner (item) or Fluid Burning type) and you can benefit from neighbour bonus, that multiplies the heat you get from the fuel, meaning you can get more power out of the same fuel.
Since the Fluid Heat source can pass fuel from one to the next, you can actually create a solid grid of them, and be able to fuel them with fluid, and get the heat out, creating the most efficient energy source in the mod, as they'll have neighbour bonus from all sides.
