In 2015, I used a Carradice Pendle, 11 litres and everything fitted in perfectly. First issue I had was at the first stop, I bought a couple of bagguettes, one for then and one for later. I found I had no spare space for the baguette so I had to flatten it. Second issue was, I had no space to rummage in the bag and to find things, I had to take things out to give myself rummaging space but I was concerned I may lose things in the dark. And the third issue was, I must have lost a good bit of time repacking the bag correctly at checkpoints or whenever I had got stuff out or putting stuff back in, otherwise I could not fasten it but I would sooner have been able to just throw things in by having some extra space. In fact, if something broke of my bike and it was valuable and repairable, I would not have had anywhere to stash it apart from lash it to the top.
This year, I will use my Super C, pack nothing more than last time and look forward to having that extra space and its only about 300 grm heavier than the Pendle, nothing.
As for a musette, I have always carried one with me on long rides, rarely use it in the UK on Audax's as I eat at the checkpoint etc usually, but on solo rides, fill it in a supermarket and eat and drink as I ride. In PBP, it was a godsend as there could be a bit of walking as you visit the various places you need to be, check in, toilets, restaurant and maybe somewhere to sleep or rest etc. Stop at checkpoint, water bottles in it along with brevet card and small wallet, walk into checkpoint and it free's your hands up to get some food etc and do other things. Planet X at the moment is selling them for 0.99p.