For me, the ship would be very much fulfilled if they decided to live together and pierce out Elena, as in Rei being the one who brings Shiho closer to her mom. Of course the age gap is a problem; the relationship of a very close, almost-family friend would already make me reach my shipping heaven. But a full-fledged romance might go just as well.
I can very much imagine Rei capable of not only setting her straight in moments of Shiho's weakness which might increase when there is no fear to overshadow it, but also capable of riling her up if she should lose herself in the emptiness after the BO is taken down. Besides I think Rei (and Hakuba as well) has a complementary personality to Shiho's more mature side. They can keep each other on their toes and support each other on rainy days due to their (more or less) common past.
That is my problem with Hakuba. He's a great guy and I'd entrust him with someone of Shiho's calibre but a) they have no plot connection and b) chances of him turning into a blushing schoolboy are higher than with Rei, esp. because I remember it happening to him to some character (was it Ran?). At that moment, I think he won't be compatible with Shiho.
That is my reason for a Sherry Bourbon ship. Just imagine the tension that could build up!
The reason why I can't imagine an Akai×Shiho is mostly because of Akai×Akemi. Shiho might be suffocated by her sister's shadow on Akai.
