If you can make or get a larger downpipe and exhaust system, that would be a lot better than stock. With my 2.5" stainless steel system, on a 94 Golf 1.9TD, I got about 2.5-3 mpg more than stock and slightly better acceleration, especially above 100km/h. Of course getting one made will probably cost $700-900 CAN. Materials for my system was about $275 and about 80 hours of my time. It was well worth it though.