Let's do the math.

Say the 8 hour flight has a 7 hour cruise at Mach 0.82 at 35,000 feet, or 473 knots. You fly 3300 nautical miles during the 7 hours.

Now, all you're asking for is a tiny change of 4 minutes per hour, or take about 30 minutes off the 3300 nautical mile distance flying time. OK. Now you need to do about 510 knots to fly the same 3300 nautical miles in 6.5 hours, or Mach 0.89, which is impossible.

edit:
I had the wrong units on some numbers: changed to knots for speed and nautical miles for distance