I never preferred HK style sights to American battle rifle sights, so the new flattop version is appealing to me so I can put on them whatever sights I like. As for the stock folding or telescopic, I think a SCAR style stock would be awesome on this thing because it folds and is adjustable, but I'd probably prefer the telescopic to the simple folding for function.
As for magazine choice, it's no question that AR mags are the most logical for Americans like us who obviously have AR15s and plenty of magazines. But I'd be willing to spend the extra on them for this rifle and get a set of the good HK M16 mags. The American M16 button magazine release operation is also much faster and more ergonomic than the European magazine release lever on the bottom, so in my mind, this is a definite improvement.
As for this not being the 551, well, just maybe it will be a better semi-auto rifle.