In other peoples opinion, Is it a good idea to increase a craft skill past 7 ranks. I have a Arch - mage with 7 suncraft, and I have all the spells I want from that school. Does putting more pts in suncraft give me more mana points or should I concentrate on some other school?

No. If you do not want any other sun spells then forget Suncraft and concentrate on other skills. The "-craft" skills have no effect on your spellpoints (the SPI attribute does). Nevertheless, answer yourself the following questions:
1. Are you really sure you do not want any other spells? Do you take into account that your spell points increase for a particular school whenever you gain a new spell in that shool and that adding a new spell is the only way of increasing your spell points unless you are playing a solo character or, perhaps, a duo?
2. Well, Suncraft is "your" magic school now (mage - Suncraft and Stonecraft). Training "your" magic school over 7 is nonsense. What about role changing? Will it be "your" magic school all the time? In the other words, is 7 for Suncraft enough?
Therefore, stop investing into Suncraft NOW and decide what to do later. You can always train in the Mages guild. Add new sun spells (even if you know that you'll never use them, sorry, this is the logic of W&W) if you feel that you do not have enough spell points in sun magic.