If I couldn't have got the CSS muffler, I would have bought the CS one. The descison to go CSS over CS was mainly based on the fact the pipe matched the GTI, so for that reason alone it was the logical choice.
But don't let a slight reduction in the pipe put you off, it's only a fraction of the overall length of the system. But if it bothers you could get a bigger pipe made and welded on the muffler, cut the pipe off the stock GTI suitcase and weld that on, or rather than cut the pipe in the usual position you could cut the stock pipe passed the elbow, close to the stock muffler, leaving only a 2 or 3 inch stub on the stock suitcase and then cut the CS muffler pipe to suit.
If you have an existing order for a CSS muffler you are in a pretty good position to ask for a decent discount on a CS one, which are a bit cheaper to start with.
There is every chance the end result will be the same, but if it turns out to be slightly quieter than the CSS, then you can do the split pipe res bypass, or delete the res completely in the usual way.
Another thing, if the CS muffler turns out to be a little quieter than the CSS, it could turn out to be the one to have, if you have a dp or plan to get one in the future.