I just spectated a rank 10 Immortal player who had bought Veil of Discord and opted for the +10 Burning Spears DPS talent. He didn't manage to get to lvl 25 for the pure damage spears, but I was curious to try this out so I loaded up a demo.
So you can amp regular Burning Spears with Kaya and Veil as expected and also the +10 DPS version. However once you get the pure damage talent only Kaya boosts the damage and Veil does nothing.
To me this is clearly a bug, but I wanted to check first.