I thought about what you wrote earlier when I was at the pool today, looking at people who were swimming along at ~55-60 seconds per 50. It wouldn't matter if some of them triple their volume, they ain't getting much quicker with the technique they've got. One guy who I've seen over the past couple of weeks is super fit, swimming heaps, but can't break 60 for 50 because he nearly touches his opposite ear with every hand entry.
So yes - whilst doubling volume can reap rewards, it's likely to help people who already swim reasonably well technique wise... and there's no way of telling if the OP swims reasonably well. It might be something as simple as they may be looking too far forward & dropping the hips as a result.
If you find a good coach to look at your stroke and find some easy fixes, the OP could be dropping 5-10 seconds without doing any more than the 8k per week they are doing.