Think I was right. Had a go at the inserts with a 1000 DXS Bronze balls. First I chronoed about 20 balls with a stock barrel and got +-10 fps. Then I did the same with insert which still allowed most of the balls to roll through and got +-20 fps. With the inserts that gripped the balls i was getting a huge variation of +-30-40 fps.
Then I did some target practice using a laser sight and found that the best grouping was with my stock barrel which was quite bigger then the balls. The smaller the insert the worse the grouping was. Most of the wild and curly shots came through the smaller insert that solidly gripped a lot of the balls.
This was by no means a controlled, accurate scientific test but it was good enough to convince me that I wasted the money I paid for the insert kit.