Same forks I have, I run 20wt in them. I did run 1" spacers but just took them out as it was a bit too stiff.
It would be hard to diagnose without trying them out. Usually slow rebound is not an issue unless the forks are packing down on you in a race....meaning they progressively lose travel because they don't have time to rebound between compressions. It's an easy thing to spot...you come into t3 hard on the brakes in bumps and you crash because the forks are bottomed out and you have no suspension.
How slow is slow? I mean can you sit there and watch them slowly extend back to full length? Or does it just "seem" slow. Keep in mind when you are compressing the forks while sitting on the bike you've got the weight of the bike and you pressing down...on rebound you just have the springs to bring them back up again. When you are just sitting on the bike pushing down on the forks rebound will always seem about twice as slow as compression.
Usually it's better to diagnose a suspected suspension problem by looking for a problem on the track, it behaves a lot differently in motion.