The Kongos were rebuilt twice. they originally had "65,000
shaft horsepower (48,000 kW), using steam provided by 36
Yarrow or Kampon
water-tube boilers, with working pressures ranging from 17.1 to 19.2
atm (1,733 to 1,945
kPa; 251 to 282
psi).
[10] The boilers, arranged in eight compartments, were mixed-firing with
fuel oil sprayed onto the coal for extra power. The ships had a stowage capacity of 4,200 long tons (4,300 t) of coal and 1,000 long tons (1,000 t) of oil, giving them a range of 8,000
nautical miles (15,000 km; 9,200 mi) at a speed of 14 knots (26 km/h; 16 mph).
[11] The battlecruisers were designed to reach a speed of 27.5 knots (50.9 km/h; 31.6 mph) and all of them exceeded that speed on their
sea trials. "
during the 20s the Japanese reboilered them with "10, 11 (
Hiei) or 16 (
Haruna) Kampon boilers, and their fuel stowage was rearranged to accommodate 2,661 long tons (2,704 t) of coal and 3,292 long tons (3,345 t) of oil. This increased their range to 8,930
nautical miles (16,540 km; 10,280 mi) at 14 knots and allowed the fore
funnel to be removed, which greatly decreased smoke interference with the
bridge and
fire-control systems. Coupled with the addition of external torpedo bulges, this reduced their speed to 26 knots (48 km/h; 30 mph) and caused the IJN to reclassify them as battleships."
The Iron Dukes, without a large increase in speed could not catch the Japanese 12 gun ships let alone (or never) the Kongo class. The Japanese had provided their ships with greatly increased main gun elevation, perhaps more than needed, but even at 20 degrees elevation they out ranged the Iron Dukes by 3-4,000 yds.
The Japanese might not have been deterred by the moderately rebuilt Iron Dukes. Since the British never really modernised the "R" class one wonders if there was enough money or dock yard capacity to upgrade even older ships.