10 knots reduction in V1 equates roughly to a decrease of 700 feet stopping distance.
At Vref minus five knots, ISA, sea level, flap 40 and 46,700 kgs the loss of lift due 5 knots (same incidence) is 7500 lbs. A two degree decrease of incidence at same IAS means 16,000 lbs loss of lift. A five degree Celsius increase in OAT will cause 3000lbs loss of lift..
Flap 30 in lieu of flap 40 from 1500 ft to touchdown saves 25kgs of fuel. 1600 kgs reduction in landing weight means trip fuel reduced by 3.5%
Info from various Boeing Airliner magazines over the years and applicable to the B737-200.