on a mk3 the vag-com allows your to monitor your ECUs output but i dont think there is much that can be done in the way of changing things. From what i understand the cheaper interfaces only allow pulling down of error codes. Im not sure if they work with the ross-tech software and allow you to log data like the genuine ross-tech one does. Im not 100% certain on it cos i sprung for the ross-tech canbus compatible one but someone with one of the cheaper ones may be able to chime in.