on prop size, increasing the first number (diameter) will *strongly* increase thrust, but only slightly affect speed. increasing the second number (pitch) will dramatically increase the speed, with only a slight increase in thrust.
This assumes the thrust is enough to overcome drag -- if the drag force slows the plane down too much a higher pitch prop will actually perform worse, since props performs best when moving at their designed speed (implied by the high pitch #). It also assumes the motor isn't bogging down under too heavy of a load. an 8x4 prop is easier to throw than an 8x6 -- if the motor is just right at 8x4, the 8x6 might be too much and the motor will spin more slowly, costing you in max speed.
but enough theory . . . what about your motor? The NTM 28-26A 1200KV, according to the specs, on a 3S pack and the 8x6 is pulling 13A -- *Well* under the spec'ed 17A max. So yes, it should give you 40-50% more speed, assuming the prop isn't too narrow to overcome the drag at higher speeds. other options?
- going to an 8x8 should be safe and might nearly double the speed from an 8x4.5.
- jumping up to a 9x4.7 should be about the same draw as the 8x6, but that won't get you much more speed over the 8x4.5.
- Jumping up to a 9x6 is iffy -- depends on the drag on the airframe. You *should* have enough head room on max current left to swing this prop, but if the 8x6 is performing well, the 9x6 won't beat the 8x6. if the 8x6 is struggling under the increased drag, you'll get significantly more thrust and closer to that 50% more speed.
Hard to say if any steeper pitch than a 9x6 or 8x8 will over-current this motor, and if you want to try one, I'd suggest using a wattmeter on a bench to make sure.