I don't feed bakery waste, but do feed fruit & veg waste (keeping within all the legalities) along with my pig nuts for the very reasons you talk about
The way I see it is bread & cakes will maintain them but it's the pig equivalent of junk food, they do far better on a balanced diet with all the vitamins & minerals needed for good growth and health. While it's possible to skrimp a bit on nuts and up the other foods for fatteners if I wanted to, I certainly wouldn't consider it for any breeding stock - I just feel they need the balance to ensure a long and productive life.
Feed companies spend hundreds of thousands of pounds researching their products, making sure they've got the right balance for growth & health, I wouldn't have a clue where to start
So rather than risk screwing up my stock (and dealing with the licensing for feed mixing) I just stick to the nuts. Depending on where you are in the country it's worth checking out East Coast Viners (Stonehaven) their rolls are coming in at around £300 the tonne - a saving of over £70 a tonne on any other prices I've been able to find (Harbro, BOCM Pauls, Carrs Billington, D & H etc) the pigs are doing well on it and the company are fantastic to deal with.......if you do contact them, please tell 'em you heard about them through the Scottish Pig Keepers Association
Thanks, Karen