Thanks Craig, my foot size is 43 (French size) so not particularly small. As said above, I like my straps to be narrow but high (so I can get far inside thanks to the height but I have control thanks to the narrow sides). From a safety point of view, I understood that most proriders do it so that they can easily get out of the strap. Currently on my 3 wave boards (Grip75, Grip82 and Tri89), I tune my back strap inserts for thin summer boot as follow: front insert is the first from the back and back insert is the second one from the back (so 1 insert tighter than normal). And the washers are tuned in. This is not tight enough for me when barefoot. But for the front strap, you cannot do that. If you try to not use standard insert position, you change the angle of the front strap. I tried to measure how wide the inserts are and it looks like its 15-15,2cm. Most people I know when they order customs will ask for 13,5-14cm even if they use boots. I guess it is difficult for you to adapt all sizes...
On a positive note, the fact that there is double insert is good for me as this should make the straps a bit more rigid which is lacking a bit currently when you make the straps very high.