Since this still doesn't have a definitive answer, I'm just going to throw my experience in and hope it gives some extra data.
I usually play balanced, and don't notice a whole lot when my opponents change strategies. RNG does make it difficult to notice any changes sometimes.
This past World Cup, I came up in the round of 8 against the Dragonriders. I was 0 for 6 against them between League and friendly matches up to that point. I did a quick browse through their past World Cup games looking for a weakness, when I noticed that they consistently played Defensive. I decided to risk everything on changing my usual Balanced strategy to match his Defensive strategy. I had 3 very capable chasers, and one of the top keepers in the game. I ended up winning that match in a big upset.
However, in the next round, I risked switching to Defensive against he Jumbucks, who stuck with Balanced. I had won against them consistently in League matches, but these were usually close matches won by my seeker. I ended up losing while being out of snitch range. Could be bad strategy, or it could be RNG.
This season, at least while I'm playing Pearl and Simona, I will be sticking with Defensive, hoping to keep the score low enough to give Gilda a chance at winning. I'll report back later in the season, but I don't think my team as a whole is competitive enough for this to make a difference.