I have a IDEfix Express in my main A1200, and one of those cheap 4-way adapters that AmigaKit sold a few years ago in my other A1200.
The speed gain mainly goes for when large files are being copied, under normal use the difference isn't really that big. If the MaxTransfer value is set to 0x1FE00 or lower on the partitions, the speed will of course not be any faster.
The IDE fix express gives some gains, not sure about the cheap 4 way adapters. But this Elbox product doesn't produce much difference in transfers.
It is a good product for cheaply adding a DVD and HDD though.