I've seen those restrictors tried before Neil, be very cautious, as all it did was cause water overheating by restricting the flow too much.
You can get different size restriction holes in them though. I'd look at making something up to make a small change to the standard size and see the difference it makes, before going to the full pinhole turbulator or whatever it is they call them.
thats the one I saw just cause overheating.
Cheers
Rob