So, as built, what speed could the Renown and Repulse make?

I did an experiment to see what the Renown and Repulse, as built, might have been able to make in 1916. My guess is about 33 knots, if the machinery was pressed hard. The report in the 1919 Jane's Fighting Ships that they had reached 41 knots is a joke. Even if they were extremely light, I find that sort of speed to be very doubtful. I might try reducing ammunition supply and fuel to see if they might have been able to faster, without compromising strength too severely. This is the Springsharp report:

Repulse, Great Britain Battlecruiser laid down 1915 (Engine 1919)

Displacement:
 28,729 t light; 29,990 t standard; 31,989 t normal; 33,588 t full load

Dimensions: Length (overall / waterline) x beam x draught (normal/deep)
 (800.00 ft / 794.00 ft) x 89.00 ft x (29.67 / 30.82 ft)
 (243.84 m / 242.01 m) x 27.13 m  x (9.04 / 9.39 m)

Armament:
      6 - 15.00" / 381 mm 42.0 cal guns - 1,920.00lbs / 870.90kg shells, 90 per gun
   Breech loading guns in turret on barbette mounts , 1915 Model
   3 x Twin mounts on centreline ends, majority forward
  1 raised mount - superfiring
      17 - 4.00" / 102 mm 45.0 cal guns - 32.27lbs / 14.64kg shells, 150 per gun
   Quick firing guns in deck and hoist mounts , 1915 Model
   4 x Twin mounts on sides, evenly spread
   3 x Triple mounts on centreline, aft deck forward
  2 raised mounts - superfiring
 Weight of broadside 12,069 lbs / 12,069 kg

Armour:
   - Belts:  Width (max) Length (avg)  Height (avg)
 Main: 6.00" / 152 mm 490.00 ft / 149.35 m 16.00 ft / 4.88 m
 Ends: 4.00" / 102 mm 300.00 ft / 91.44 m 12.00 ft / 3.66 m
   4.00 ft / 1.22 m Unarmoured ends
   Main Belt covers 95 % of normal length
   Main belt does not fully cover magazines and engineering spaces

   - Torpedo Bulkhead:
  1.50" / 38 mm 490.00 ft / 149.35 m 29.00 ft / 8.84 m

   - Gun armour: Face (max) Other gunhouse (avg) Barbette/hoist (max)
 Main: 9.00" / 229 mm 5.00" / 127 mm  7.00" / 178 mm
 2nd: 3.00" / 76 mm       -   2.00" / 51 mm

   - Armour deck: 3.00" / 76 mm, Conning tower: 9.00" / 229 mm

Machinery:
 Oil fired boilers, steam turbines, 
 Geared drive, 4 shafts, 179,547 shp / 133,942 Kw = 33.00 kts
 Range 6,500nm at 15.00 kts
 Bunker at max displacement = 3,598 tons

Complement:
 1,195 - 1,554

Cost:
 £4.081 million / $16.323 million

Distribution of weights at normal displacement:
 Armament: 1,642 tons, 5.1 %
 Armour: 7,670 tons, 24.0 %
    - Belts: 2,532 tons, 7.9 %
    - Torpedo bulkhead: 789 tons, 2.5 %
    - Armament: 1,458 tons, 4.6 %
    - Armour Deck: 2,696 tons, 8.4 %
    - Conning Tower: 195 tons, 0.6 %
 Machinery: 6,376 tons, 19.9 %
 Hull, fittings & equipment: 13,602 tons, 42.5 %
 Fuel, ammunition & stores: 3,260 tons, 10.2 %
 Miscellaneous weights: 0 tons, 0.0 %

Overall survivability and seakeeping ability:
 Survivability (Non-critical penetrating hits needed to sink ship):
   41,011 lbs / 18,602 Kg = 24.3 x 15.0 " / 381 mm shells or 4.8 torpedoes
 Stability (Unstable if below 1.00): 1.14
 Metacentric height 5.4 ft / 1.6 m
 Roll period: 16.1 seconds
 Steadiness - As gun platform (Average = 50 %): 55 %
   - Recoil effect (Restricted arc if above 1.00): 0.93
 Seaboat quality  (Average = 1.00): 1.11

Hull form characteristics:
 Hull has low quarterdeck ,
   a ram bow and a cruiser stern
 Block coefficient (normal/deep): 0.534 / 0.540
 Length to Beam Ratio: 8.92 : 1
 'Natural speed' for length: 28.18 kts
 Power going to wave formation at top speed: 53 %
 Trim (Max stability = 0, Max steadiness = 100): 50
 Bow angle (Positive = bow angles forward): -3.00 degrees
 Stern overhang: -4.00 ft / -1.22 m
 Freeboard (% = length of deck as a percentage of waterline length):
    Fore end,  Aft end
    - Forecastle: 20.00 %,  31.00 ft / 9.45 m,  28.00 ft / 8.53 m
    - Forward deck: 30.00 %,  28.00 ft / 8.53 m,  27.00 ft / 8.23 m
    - Aft deck: 18.00 %,  27.00 ft / 8.23 m,  27.00 ft / 8.23 m
    - Quarter deck: 32.00 %,  19.00 ft / 5.79 m,  19.00 ft / 5.79 m
    - Average freeboard:  25.03 ft / 7.63 m

Ship space, strength and comments:
 Space - Hull below water (magazines/engines, low = better): 117.1 %
  - Above water (accommodation/working, high = better): 211.5 %
 Waterplane Area: 48,585 Square feet or 4,514 Square metres
 Displacement factor (Displacement / loading): 110 %
 Structure weight / hull surface area: 183 lbs/sq ft or 896 Kg/sq metre
 Hull strength (Relative):
  - Cross-sectional: 0.96
  - Longitudinal: 1.34
  - Overall: 1.00
 Hull space for machinery, storage, compartmentation is cramped
 Room for accommodation and workspaces is excellent