Look again at the Y1B-17, maybe they could do a a bit better, but another 15-25mph doesn't change anything.
Please remember that the drag goes up with the square of the speed and actual power required because of the time element, goes up even more.
To get the Y1B-17 up to 282mph (assuming it did the 256mph with 775hp) would require 1030hp according to the cube law.
Lets assume you can shift the wing higher on the Y1B-17 and not get stuck with the real B-17s bomb bay problems. Going very far with 8-10,000lbs isn't really going to happen.
Data card for the Halifax I ;
http://www.wwiiaircraftperformance.org/Halifax/Halifax_I_ADS.jpg
larger, heavier, more power and please note ranges are for most economical speed which is 195mph at 15,000ft. Also please note the difference in Ceiling between max weight and mean weight. Also note the take off distance.
The Halifax II took off in 200yds less distance despite weighing 1,000lbs more, max weight ceiling went up 3,000ft.
I would also note that Wiki (or who ever wrote it) didn't transfer the source materiel quite right for the Sunderland, the 34,500lb weight is
tare empty which is a bit different than some other empty weights. tare weight does NOT include such things as guns or radios. Service load (including 11 man crew) was 7060lbs. Petrol (2,155 gallons) was 15,540lbs, oil (100 gal) 900lbs.
The 178mph cruise speed is the
MAX economical cruise speed, equivalent the Halifax doing 233mph instead of 195mph. It is also at 5,000ft which means the supercharger is in low gear. Granted the air is thicker and has more drag but the engine is giving about 10% more power to the prop because it is not driving the high speed supercharger gear.
The Sunderland is just so different it doesn't really make a good comparison even it it does use the same engines.
here are some numbers for very early B-24s without turbos, gun turrets, armor or self sealing fuel tanks.
Four Pratt Whitney R-1830-33 (S3C4-G) Twin Wasp fourteen-cylinder air-cooled radials rated at 1200 hp for takeoff and 1000 hp at 14,500 feet. Performance: Maximum speed 273 mph at 15,000 feet. Cruising speed 186 mph. Landing speed 90 mph. Service ceiling 31,500 feet. An altitude of 10,000 feet could be attained in 6 minutes. Range was 3000 miles with a 2500 pound bombload. Maximum range was 4700 miles. Weights: 27,500 pounds, empty, 38,360 pounds gross, 46,400 pounds maximum. Dimensions: Wingspan 110 feet 0 inches, length 63 feet 9 inches, height 18 feet 8 inches, wing area 1048 square feet.
Again from Joe Baugher's website. This for the LB-30A version of the B-24 and it didn't fly until Jan 1941. It used sealed portions of wing for fuel tanks (wet wing, not self sealing) and could not be used for combat. I would also note that both the performance figures here and for the Y1B-17 would much more likely than not, be for the "gross" weight and not the
maximum and would more closely correspond to the British mean weight. I mean less than 11,000lbs between empty and gross and the plane is 'supposed' to hold 8,000lb of bombs? take out even 1200lbs for a crew if 6 and you don't have much left for gas and oil.
designers were trying but you just don't get a very useable 4 engine bomber with engines around 1000hp.
Data card from Mike Williams site.