With a 1000kv motor, it would likely be happier with a 9X6 or 10X5.5 prop. Where its a sail plane, a lower kv motor will be slightly more efficient, but to get the same or maximum thrust for it, you generally want a slower, or at least larger prop for peak performance.
Leaving a 8X4 prop on it will not degrade performance if your happy with the way it flies. Throwing over propping an electric motor can make a lot of differance on the amprage and can burn up the motor if your not careful. Throwing a smaller prop on shouldn't hurt it at all, and will lower your amprage and draw on the motor, equalling less power used (generally) and longer flight times.
Being as it is a larger sail plane, you might be a lot happier with a larger slower prop to get it up in the air so you can thermal quickly. If your going to use the motor to fly the plane constantly, i'd probably keep it as is.
My slowfly motor recommended at 11X7 for best performance, but there was a noticeable differance running a 11X8.5, and put me right at the maximum ratings, with more thrust.
On my dad's advance, his motor was only pulling 230 watts with what we thought was the recommended prop. Turns out we wrote down the wrong specs. Just by going from a 11X5 to a 12X8 made all the differance in the world on 3 cells, and took it from a very under powered plane to being reasonable performance with limited verticle andpulling 340 or so watts running two 1800's in parelell. Throwing a 4 cell 2200mah in it gave much better performance on the same prop, and still being well under ratings. It pulled about 425 watts on the same 12X8 prop, and minus shorter flight times, was a much better performer, but still way short of the 550w it was capable of.
We decided that we couldn't run the proper 13 to 14" props to get better performance out of the 3 cell because of prop clearance. We figured if e-flight recommended a 12X6 on their motor for this plane, we should be able to get away with the 13" prop on this plane. Instead we had well then a fingertips clearance, and decided not to try it.
Right now we are thinking of going to a slightly larger 1100kv motor for it, to try for longer flight times and better performance on 3 cells so we will be able to run a smaller prop and not have to re-buy batteries. With this other motor we should be able to get into the 700w range on his 4lb airframe, which should give awesome verticle when we want it, and hopefully better flight times when we don't.
Right now we are getting a 6 minute flight or so on the 4 cell 2200 and 7-10 minutes on two 1800 mah 3 cells, giving us the equivilent of a 3600 mah battery. I think we are working the motor too hard with 3 cells, hence the similar flight times out of the much smaller 4 cell. I think we could get better thrust and similar to lower amprage if we could fit the larger prop. Right now the plane flies steady at 3/4 throttle with a slight climb, and needs a bit over 1/2 to stay in the air.
On my slowfly and sport motor, I can fly either on less then 1/4 throttle and gain altitude.
Make sure your motor doesn't get hot to the touch after a couple of power runs until you can get a watt meter setup. However, I feell you should be safe running such a small prop on that motor.