There is lots of evidence for both, probably less under the term portion control but lots more for calorie restriction. Here's one for portion control
http://www.ncbi.nlm.nih.gov/pubmed/17592101
One for GI
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=7892481
For a totally contrary but seemingly extremely successful trial of a rather whacky sounding diet look up the results of the 6 momth Ma Pi 2 macrobiotic diet ( brown rice, beans and seaweed ! the absolute opposite of a LCHF diet ) in Cuba
http://www.ncbi.nlm.nih.gov/pubmed/21483296
Seriously one can find evidence for many diets, the difficulty is sorting out the wheat from the chaff. By their very nature diet studies are hard to control and for anything over 6 months it becomes increasingly difficult to motivate people to stick to their assigned diets. (wonder how long people would stick to the Ma Pi diet outside of the metabolic ward) Many trials also are weight loss trials with results for glucose levels and other things 'thrown' in. That doesn't help people with any type of diabetes who don't need to lose weight nor those who have lost it and need to maintain weight and glucose levels.
Often the results with any intervention are less than spectacular. ( though personally, I think that there have been some good results from longer term Med diet ones recently)
I think you say that you come from Sweden. It's worth reading the report of your own SBU scientific committee. They emphasise the lack of quality in most trials . I can give you the link to the English summary but it doesn't have references. I'm sure that the full document will have.
http://www.sbu.se/upload/Publikationer/Content1/1/mat_diabetes_eng_smf_110517.pdf