Maximum cooking time for beef joint in slow cooker?
I do think there is a maximum to the duration of stewing a cut of beef. There is in my opinion such a thing as too tender and soft. If you plan to cook the meat as one large chunk you will have to add a little to your normal stewing time, but I would not go as high as 15 hours.

Cooking and re-heating is quite an interesting option though, as quite a few dishes benefit from this in terms of flavour. Perhaps a general question on this subject, or a more directly relevant one would be of interest to you.

For me, the smaller the size of your chunks of meat, the more you should stick to your normal stewing time of 8 hours and the more you could consider making it a day early to enhance the taste.