I had a discussion with a retailer in th HB industry and here's what resulted. if you use a can that "promotes" 1kg of "sugar", dont add the "sugar" and make to 11.5 litres..excellent beer..the sugar doesn't add flavour ,it just ups the alc level and thins out the body of the beer, hop level is the same. Therefore the same goes for the MS cans....add your usual adjuncts and make to 22.5/23ltr. The only reason they stipulate 11.5ltr and no additives; is to keep it in a ball park of flavour of the JS beers and make you, THE CONSUMER,pay near double the price of any decent can on the market.Only when you add malt and other adjuncts that leave flavour residuals/unfermentables does it change the hopping rates/mouthfeel of the can...which can be easily altered with extra hop additions and proper additives. The trick is matching the hops used in the can and achieving the desired hop levels without starting IBU's of the can.
Something to muse anyway