Not only is the SDI terrible it isn't particularly economic either. We had one in the works Caddy van (basically a Golf based van). While it was in for a service we had the TDI version as a loaner, and after driving the TDI back to the garage and the SDI back he turned around and complained that this van was obviously faulty. The VW dealer just told him, no the SDI really is that crap.
I don't know why they even bothered making it. Even when completely unladen it took forever to get that thing up to 70 on the motorways, and once there you really didn't want to slow down again so the drivers would do some really naughty things to try and keep the speed up such as inappropriately fast cornering, tailgating people who have pulled out in front hoping they'd pull in again etc. etc.