It's the erase blocks that cause the real problems. If a page is 8KB or 16KB then raw writes would hit the IOPS cap before the bandwidth cap, but that wouldn't be a major source of issues. With erase blocks at a megabyte or larger, they introduce enormously complex remapping and garbage collection requirements. You can't do a whole lot about those problems with a block device API.

It helps a bit, but keep in mind that just because your filesystem is doing most of its allocation in 64kB chunks, doesn't mean it is issuing only 64kB IOs to the drive itself.

Additionally it doesn't mean that your 64KB chunks aligned to your current partition are also aligned with the 64KB boundary of the underlying block device.